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

[Anton Korobeynikov via llvm-commits]

asl wrote:

We can certainly upgrade the bot message and make it a required check. However, this would cover only maybe half of the issue. The bot checks for email addresses used in commits. However, the email address is rewritten during "Squash & Merge" and it seems we neither cannot control nor prevent this.

Looks like the only solution is to disable direct merges at all? And delegate to some bot?

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