Add CLA note to tree hygiene (#172475)

Adds a note about the CLA to the steps to contributing code to Flutter,
including suggested language for how this applies to use of generative
tools.

docs/contributing/Tree-hygiene.md

diff --git a/docs/contributing/Tree-hygiene.md b/docs/contributing/Tree-hygiene.md
index eef9f8b..f4c9064 100644
--- a/docs/contributing/Tree-hygiene.md
+++ b/docs/contributing/Tree-hygiene.md
@@ -61,6 +61,12 @@
 6. Submit this branch as a PR to the relevant Flutter repository.
    _(See also: [Signing commits](./Signing-commits.md))_
 
+   All submissions to Google Open Source projects need to follow
+   [Google’s Contributor License Agreement (CLA)](https://cla.developers.google.com/),
+   in which contributors agree that their contribution is an original work of authorship.
+   This doesn’t prohibit the use of coding assistance tools, but what’s submitted does
+   need to be a contributor’s original creation.
+
 7. Get your code reviewed (see below). You should probably reach out to the relevant
    expert(s) for the areas you touched and ask them to review your PR directly.
    GitHub sometimes recommends specific reviewers; if you're not sure who to ask,