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

[David Spickett via llvm-commits]

================
@@ -0,0 +1,27 @@
+name: PR Greeter
+
+on:
+  pull_request_target:
+   types: [ opened ]
+
+permissions:
+  pull-requests: write
----------------
DavidSpickett wrote:

Done.

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