Clarify CONTRIBUTING.md

PiperOrigin-RevId: 466027072

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d8c6e48..cb06def 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -26,6 +26,9 @@
 3.  Email a link to your pull request to <quiche-contribution@google.com>.
 4.  An engineer will review your pull request and merge it internally.
 
+Note: if you are a Google engineer with access to google3, please submit a CL to
+google3 directly.
+
 ## Community Guidelines
 
 This project follows