Add Windows support to CONTRIBUTING.md (flutter#3825)

CONTRIBUTING.md

@@ -6,13 +6,16 @@ Contributing to the Flutter engine
 Things you will need
 --------------------
 
- * Linux or Mac OS X. (Windows is not yet supported.)
+ * Linux, Mac OS X, or Windows
+     * Windows doesn't support cross-compiling artifacts for Android or iOS
+     * Linux doesn't support cross-compiling artifacts for iOS
  * git (used for source version control).
  * An IDE. We recommend [IntelliJ with the Flutter plugin](https://flutter.io/intellij-ide/) or Xcode.
  * An ssh client (used to authenticate with GitHub).
  * Chromium's [depot_tools](http://www.chromium.org/developers/how-tos/install-depot-tools) (make sure it's in your path). We use the `gclient` tool from depot_tools.
  * Python (used by many of our tools, including 'gclient').
- * curl and unzip (used by `gclient sync`).
+ * On Mac OS X and Linux: curl and unzip (used by `gclient sync`).
+ * On Windows: Visual Studio (required for non-Googlers only).
 
 You do not need [Dart](https://www.dartlang.org/downloads/linux.html) installed, as a Dart tool chain is automatically downloaded as part of the "getting the code" step. Similarly for the Android SDK, it's downloaded by the `gclient sync` step below.
 
@@ -52,6 +55,7 @@ target_os = ["android"]
  * If you're on Linux, run `sudo ./build/install-build-deps.sh`
  * If you're on Mac, install Oracle's Java JDK, version 1.7 or later.
  * If you're on Mac, install `ant`: `brew install ant`
+ * If you're on Windows, install Visual Studio (non-Google developers only)
  * If you're planning on working on the [buildroot](https://github.com/flutter/buildroot) repository as well, and have a local checkout of that repository, run the following commands in the `src` directory to update your git remotes accordingly:
 
  ```bash
@@ -148,6 +152,16 @@ To run the tests, you'll also need to clone [the main Flutter repository](https:
 See [the instructions for contributing](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md)
 to the main Flutter repository for detailed instructions.
 
+### Desktop (gen_snapshot for Windows)
+
+You can only build selected binaries on Windows (mainly `gen_snapshot`).
+
+* Make sure you have Visual Studio installed (non-Googlers only).
+* `git pull upstream master` in `src/flutter` to update the Flutter Engine repo.
+* `gclient sync` to update your dependencies.
+* `python .\flutter\tools\gn [--unoptimized] --runtime-mode=[debug|profile|release] [--android]` to prepare your build files.
+* `ninja -C .\out\<dir created by previous step> gen_snapshot` to build.
+
 ### Building all the builds that matter on Linux and Android
 
 The following script will update all the builds that matter if you're developing on Linux and testing on Android and created the `.gclient` file in `~/dev/engine`: