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

[Reid Kleckner via llvm-commits]

rnk wrote:

In https://github.com/llvm/llvm-project/pull/136198 , I rewrote a section of the developer policy to try to cover RFCs. Aaron reviewed it. :) Blame says the original text was by @isanbard , but I retitled the section from "Making a Major Change" to "Proposing Major Changes (RFCs)" to try to capture most of what you added.

I think it's worth splitting our the RFC process into it's own doc, but I want to make sure everything is cross-referenced and to incorporate any ideas or insights I had into the new doc.

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