[llvm] [GitHub] Add Greeting comment to new contributor's PRs with useful information (PR #72249)

[David Spickett via llvm-commits]

DavidSpickett wrote:

It's sufficiently different it didn't make sense to do as a fixup here, so opened https://github.com/llvm/llvm-project/pull/72384 for the version where we always know what the job order is.

https://github.com/llvm/llvm-project/pull/72249