README: encourage issues and proposals before changes

Text verbatim from https://go.dev/doc/contribute

Change-Id: Iefdcf9e9f771b9e55601bf9c9b59e20593b4573a
GitHub-Last-Rev: ba45caadaf
GitHub-Pull-Request: golang/oauth2#632
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/471281
Run-TryBot: Matthew Hickford <hickford@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Matthew Hickford <hickford@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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
@@ -29,8 +29,12 @@ 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.
-
 The main issue tracker for the oauth2 repository is located at
 https://github.com/golang/oauth2/issues.
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html. In particular:
+
+* Excluding 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 code owners are listed at [dev.golang.org/owners](https://dev.golang.org/owners#:~:text=x/oauth2).