README: encourage issues and proposals before changes

Author: hickford

README.md

@@ -19,7 +19,7 @@ See pkg.go.dev for further documentation and examples.
 * [pkg.go.dev/golang.org/x/oauth2](https://pkg.go.dev/golang.org/x/oauth2)
 * [pkg.go.dev/golang.org/x/oauth2/google](https://pkg.go.dev/golang.org/x/oauth2/google)
 
-## Policy for new packages
+## Policy for new endpoints
 
 We no longer accept new provider-specific packages in this repo if all
 they do is add a single endpoint variable. If you just want to add a
@@ -30,7 +30,10 @@ package.
 ## Report Issues / Send Patches
 
 This repository uses Gerrit for code changes. To learn how to submit changes to
-this repository, see https://golang.org/doc/contribute.html.
+this repository, see https://golang.org/doc/contribute.html. In particular:
+
+* Excluding very trivial changes, all contributions should be connected to an existing issue.
+* API changes must go through the [change proposal process](https://go.dev/s/proposal-process) before they can be accepted.
 
 The main issue tracker for the oauth2 repository is located at
 https://github.com/golang/oauth2/issues.