[llvm] Document the community RFC process (PR #116386)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> Hi folks, do we wish to push this PR to main/gather more feedback? I think this text adds good explanation on when we accept/reject RFC and what are "symptoms" of a rejected RFC.

Thank you for reviving this thread, it fell off my radar entirely! I've rebased the PR and made some modifications to hopefully address the concerns from @nikic.

https://github.com/llvm/llvm-project/pull/116386