[llvm] [CI] Add mention of LLVM Developer Policy in email-check message (NFC) (PR #143300)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Seems reasonable enough to me. If there's information in the Discourse thread not in the documentation, it should be moved over.

We should probably drop the Discourse thread link altogether at some point.

https://github.com/llvm/llvm-project/pull/143300