[llvm] [GitHub] Add greeting comment to opened PRs from new contributors (PR #72384)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> What's the difference between FIRST_TIME and FIRST_TIME_CONTRIBUTOR?

Documented in:
https://docs.github.com/en/graphql/reference/enums#commentauthorassociation

FIRST_TIMER is new to GitHub itself, so if you wanted to you could link to more info than someone who is just new to the repository I guess. We cater to both in the docs we have so I checked for either.

https://github.com/llvm/llvm-project/pull/72384