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

[David Spickett via llvm-commits]

DavidSpickett wrote:

While these configurations are pretty fragile and a pain to write, it's readable enough once it works, so I'd prefer to go with this solution. Easier to control notifications this way.

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