add internal comment to point to .github repo (#91055)

diff --git a/CODE_OF_CONDUCT.md b/CODE_OF_CONDUCT.md
index 33ac1da..5edbd60 100644
--- a/CODE_OF_CONDUCT.md
+++ b/CODE_OF_CONDUCT.md
@@ -1,3 +1,5 @@
+<!-- when updating this file also update https://github.com/flutter/.github/blob/main/CODE_OF_CONDUCT.md -->
+
 # Code of conduct
 
 We expect Flutter's contributors to act professionally and respectfully, and