Document Rails upgrade steps

Adds notes about things we need to keep in sync when upgrading Rails

doc/development/gemfile.md

@@ -18,3 +18,16 @@ dependencies and build times.
 ## License compliance
 
 Refer to [licensing guidelines](licensing.md) for ensuring license compliance.
+
+## Upgrade Rails
+
+When upgrading the Rails gem and its dependencies, you also should update the following:
+
+- The [Gemfile in the `qa` directory](https://gitlab.com/gitlab-org/gitlab/-/blob/master/qa/Gemfile).
+- The [Gemfile in Gitaly Ruby](https://gitlab.com/gitlab-org/gitaly/-/blob/master/ruby/Gemfile),
+  to ensure that we ship only one version of these gems.
+
+You should also update NPM packages that follow the current version of Rails:
+
+- `@rails/ujs`
+- `@rails/actioncable`