doc: update quickstart README files (googleapis#4980)

Apply some of the lessons from the latest review of the Pub/Sub
quickstart README to the other libraries.

google/cloud/bigtable/quickstart/README.md

@@ -5,6 +5,14 @@ C++ client library in your own project. These instructions assume that you have
 some experience as a C++ developer and that you have a working C++ toolchain
 (compiler, linker, etc.) installed on your platform.
 
+## Before you begin
+
+To run the quickstart examples you will need a working Google Cloud Platform
+(GCP) project, as well as a Cloud Bigtable instance and table.
+The [Cloud Bigtable quickstart][bigtable-quickstart] covers the necessary
+steps in detail. Make a note of the GCP project id, the instance id, and the
+table id as you will need them below.
+
 ## Configuring authentication for the C++ Client Library
 
 Like most Google Cloud Platform (GCP) services, Cloud Bigtable requires that
@@ -33,6 +41,9 @@ https://cloud.google.com/docs/authentication/production
 
 ## Using with Bazel
 
+> :warning: If you are using Windows or macOS there are additional instructions
+> at the end of this document.
+
 1. Install Bazel using [the instructions][bazel-install] from the `bazel.build`
    website.
 
@@ -43,17 +54,21 @@ https://cloud.google.com/docs/authentication/production
    bazel build ...
    ```
 
+   Note that Bazel automatically downloads and compiles all dependencies of the
+   project. As it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Run the example, change the place holder to appropriate values:
 
    ```bash
    bazel run :quickstart -- [GCP PROJECT] [CLOUD BIGTABLE INSTANCE] [CLOUD BIGTABLE TABLE]
    ```
 
-If you are using Windows or macOS there are additional instructions at the end
-of this document.
-
 ## Using with CMake
 
+> :warning: If you are using Windows or macOS there are additional instructions
+> at the end of this document.
+
 1. Install CMake. The package managers for most Linux distributions include a
    package for CMake. Likewise, you can install CMake on Windows using a package
    manager such as [chocolatey][choco-cmake-link], and on macOS using
@@ -68,6 +83,9 @@ of this document.
    ./vcpkg install google-cloud-cpp
    ```
 
+   Note that, as it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Configure CMake, if necessary, configure the directory where you installed
    the dependencies:
 
@@ -83,9 +101,6 @@ of this document.
    .build/quickstart [GCP PROJECT] [CLOUD BIGTABLE INSTANCE] [CLOUD BIGTABLE TABLE]
    ```
 
-If you are using Windows or macOS there are additional instructions at the end
-of this document.
-
 ## Platform Specific Notes
 
 ### macOS
@@ -132,3 +147,4 @@ set GRPC_DEFAULT_SSL_ROOTS_FILE_PATH=%cd%\roots.pem
 [cmake-download-link]: https://cmake.org/download/
 [bazel-grpc-macos-bug]: https://github.com/bazelbuild/bazel/issues/4341
 [authentication-quickstart]: https://cloud.google.com/docs/authentication/getting-started 'Authentication Getting Started'
+[bigtable-quickstart]: https://cloud.google.com/bigtable/docs/quickstart-cbt

google/cloud/spanner/quickstart/README.md

@@ -5,6 +5,14 @@ client library in your own project. These instructions assume that you have
 some experience as a C++ developer and that you have a working C++ toolchain
 (compiler, linker, etc.) installed on your platform.
 
+## Before you begin
+
+To run the quickstart examples you will need a working Google Cloud Platform
+(GCP) project, as well as a Cloud Spanner instance and database.
+The [Cloud Spanner quickstart][spanner-quickstart-link] covers the necessary
+steps in detail. Make a note of the GCP project id, the instance id, and the
+database id as you will need them below.
+
 ## Configuring authentication for the C++ Client Library
 
 Like most Google Cloud Platform (GCP) services, Cloud Spanner requires that
@@ -31,6 +39,9 @@ https://cloud.google.com/docs/authentication/production
 
 ## Using with Bazel
 
+> :warning: If you are using Windows or macOS there are additional instructions
+> at the end of this document.
+
 1. Install Bazel using [the instructions][bazel-install] from the `bazel.build`
    website.
 
@@ -41,22 +52,26 @@ https://cloud.google.com/docs/authentication/production
    bazel build ...
    ```
 
+   Note that Bazel automatically downloads and compiles all dependencies of the
+   project. As it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Run the example, change the place holder to appropriate values:
 
    ```bash
    bazel run :quickstart -- [GCP PROJECT] [CLOUD SPANNER INSTANCE] [CLOUD SPANNER DATABASE]
    ```
 
-If you are using Windows or macOS there are additional instructions at the end
-of this document.
-
 ## Using with CMake
 
+> :warning: If you are using Windows or macOS there are additional instructions
+> at the end of this document.
+
 1. Install CMake. The package managers for most Linux distributions include a
    package for CMake. Likewise, you can install CMake on Windows using a package
    manager such as [chocolatey][choco-cmake-link], and on macOS using
    [homebrew][homebrew-cmake-link]. You can also obtain the software directly
-   from the [cmake.org][https://cmake.org/download/].
+   from the [cmake.org](https://cmake.org/download/).
 
 2. Install the dependencies with your favorite tools. As an example, if you use
    [vcpkg](https://github.com/Microsoft/vcpkg.git):
@@ -66,6 +81,9 @@ of this document.
    ./vcpkg install google-cloud-cpp
    ```
 
+   Note that, as it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Configure CMake, if necessary, configure the directory where you installed
    the dependencies:
 
@@ -81,9 +99,6 @@ of this document.
    .build/quickstart [GCP PROJECT] [CLOUD SPANNER INSTANCE] [CLOUD SPANNER DATABASE]
    ```
 
-If you are using Windows or macOS there are additional instructions at the end
-of this document.
-
 ## Platform Specific Notes
 
 ### macOS

google/cloud/storage/quickstart/README.md

@@ -5,9 +5,17 @@ library in your own project. These instructions assume that you have some
 experience as a C++ developer and that you have a working C++ toolchain
 (compiler, linker, etc.) installed on your platform.
 
+## Before you begin
+
+To run the quickstart examples you will need a working Google Cloud Platform
+(GCP) project and an existing bucket.
+The [GCS quickstarts](https://cloud.google.com/storage/docs/quickstarts) cover
+the necessary steps in detail. Make a note of the GCP project id and the bucket
+name as you will need them below.
+
 ## Configuring authentication for the C++ Client Library
 
-Like most Google Cloud Platform (GCP) services, GCS requires that
+Like most Google Cloud Platform services, GCS requires that
 your application authenticates with the service before accessing any data. If
 you are not familiar with GCP authentication please take this opportunity to
 review the [Authentication Overview][authentication-quickstart]. This library
@@ -41,6 +49,10 @@ https://cloud.google.com/docs/authentication/production
    bazel build ...
    ```
 
+   Note that Bazel automatically downloads and compiles all dependencies of the
+   project. As it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Run the example, change the place holder to appropriate values:
 
    ```bash
@@ -70,6 +82,9 @@ https://cloud.google.com/docs/authentication/production
    ./vcpkg install google-cloud-cpp
    ```
 
+   Note that, as it is often the case with C++ libraries, compiling these
+   dependencies may take several minutes.
+
 3. Configure CMake, if necessary, configure the directory where you installed
    the dependencies: