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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Just a +1 for the general direction. The CoC Committee has encountered this problem recently too - no ability to privately communicate with a community member/contributor if they don't have a public email address associated with their github account.

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