actions: Add note about not overloading Flathub ci

docs/02-for-app-authors/09-github-actions.md

@@ -8,6 +8,13 @@ https://github.com/flathub-infra/flatpak-github-actions
 
 ## Using GitHub actions on your Flathub repository
 
+:::note
+Please note that any custom GitHub actions that commits or pushes to PRs
+or branches should be run in a controlled manner and only when necessary.
+Since every such push creates a new build, too many triggers can overload
+the Flathub build infrastructure.
+:::
+
 If your application is hosted in the Flathub GitHub organization, you can use these actions, all others are not allowed:
 
 1. Any action from the flathub organization