[llvm] Document the requirement that commits have a public email address (PR #109318)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

Are we gonna "upgrade" the bot message from a soft message to more of a requirement?
Considering the current dynamic since I raised this, I just ignore the bot and merge PRs (like most people it seems) at the moment.

https://github.com/llvm/llvm-project/pull/109318