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

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

Another reason why this is potentially becoming more important in the relatively near future is because the proposed governance process intends to use GitHub APIs to verify contributors for voter eligibility and as a way to send ballots/voting instructions to folks. That's not what's driving this PR, but it is what got me motivated to post it.

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