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

[Bryce Wilson via llvm-commits]

Bryce-MW wrote:

I had a similar issue at work where we wanted a check on the final commit created by "Squash & Merge" and not just the earlier commits. We didn't end up going with it but the solution I came up with was using GitHub merge queues and having just that one check enabled

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