docs: update CONTRIBUTING.md with some PR setup info (#2619)

Adds an explicit mention to enable "Allow edits by maintainers" when
setting up the pull request.  In PR #2574 (external contribution) it was
discovered this cannot be added after the fact for org-based forks, only
user-based.

As there's not a great reference link for this in the github docs that
doesn't descend into a bunch of organization permissions management, I'm
not calling this specific circumstance explicitly.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 109ca5c..056a7a4 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -21,7 +21,7 @@
     `cd google-cloud-go`
 
 1. Fork the repo.
-   
+
 1. Set your fork as a remote:
     `git remote add fork git@github.com:GITHUB_USERNAME/google-cloud-go.git`
 
@@ -36,6 +36,9 @@
 
 1. Send a pull request with your changes.
 
+   To minimize friction, consider setting `Allow edits from maintainers` on the
+   PR, which will enable project committers and automation to update your PR.
+
 1. A maintainer will review the pull request and make comments.
 
    Prefer adding additional commits over amending and force-pushing since it can