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

[Tom Stellard via llvm-commits]

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

The convention we've been using for permissions is to set `contents: read` for the whole file and then write permissions like this are added to the job.  See https://github.com/llvm/llvm-project/blob/main/.github/workflows/release-binaries.yml for example.

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