add internal comment to point to .github repo (#91052)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 247d79c..edcb01a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,3 +1,5 @@
+<!-- when editing this file also update https://github.com/flutter/.github/blob/main/CONTRIBUTING.md -->
+
 Contributing to Flutter
 =======================