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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I've tested this looking for the `OWNER` condition on my fork and it seems to work. `FIRST_TIME_CONTRIBUTOR` might imply `FIRST_TIMER`, but the docs don't say either way and being explicit is better anyway.

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