docs: revert RELEASING.md to non-releasetool

Reverts RELEASING.md to roughly 64a2037
plus some changes.

Reason: releasetool no longer works for us.

Change-Id: Iade5f98828db7516598d15edb2fedf53a8188279
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40275
Reviewed-by: kokoro <[email protected]>
Reviewed-by: Chris Broadfoot <[email protected]>

RELEASING.md

@@ -1,47 +1,17 @@
-# How to Create a New Release
+# How to Release this Repo
 
-## Prerequisites
-
-Install [releasetool](https://github.com/googleapis/releasetool).
-
-## Create a release
-
-1. `cd` into the root directory, e.g., `~/go/src/cloud.google.com/go`
-1. Checkout the master branch and ensure a clean and up-to-date state.
-    ```
-    git checkout master
-    git pull --tags origin master
-    ```
-1. Run releasetool to generate a changelog from the last version. Note,
-   releasetool will prompt if the new version is a major, minor, or patch
-   version.
-    ```
-    releasetool start --language go
-    ```
-1. Format the output to match CHANGES.md.
-1. Submit a CL with the changes in CHANGES.md. The commit message should look
-   like this (where `v0.31.0` is instead the correct version number):
-    ```
-    all: Release v0.31.0
-    ```
-1. Wait for approval from all reviewers and then submit the CL.
-1. Return to the master branch and pull the release commit.
-    ```
-    git checkout master
-    git pull origin master
-    ```
-1. Tag the current commit with the new version (e.g., `v0.31.0`)
-    ```
-    releasetool tag --language go
-    ```
-1. Publish the tag to GoogleSource (i.e., origin):
-    ```
-    git push origin $NEW_VERSION
-    ```
-1. Visit the [releases page][releases] on GitHub and click the "Draft a new
-   release" button. For tag version, enter the tag published in the previous
-   step. For the release title, use the version (e.g., `v0.31.0`). For the
-   description, copy the changes added to CHANGES.md.
-
-
-[releases]: https://github.com/googleapis/google-cloud-go/releases
+1. Determine the current release version with `git tag -l`. It should look
+   something like `vX.Y.Z`. We'll call the current version `$CV` and the new
+   version `$NV`.
+1. On master, run `git log $CV..` to list all the changes since the last
+   release.
+1. Edit `CHANGES.md` to include a summary of the changes.
+1. Mail the CL containing the `CHANGES.md` changes. When the CL is approved,
+   submit it.
+1. Without submitting any other CLs:
+   a. Switch to master.
+   b. `git pull`
+   c. Tag the repo with the next version: `git tag $NV`.
+   d. Push the tag: `git push origin $NV`.
+1. Update [the releases page](https://github.com/googleapis/google-cloud-go/releases)
+   with the new release, copying the contents of the CHANGES.md.