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

[Aaron Ballman via llvm-commits]

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+=================================
+Request For Comment (RFC) process
+=================================
+
+.. contents::
+   :local:
+   :depth: 1
+
+Introduction
+============
+Substantive changes to LLVM projects need to be acceptable to the wider
+community, which requires gaining community consensus to adopt the changes.
+This is done by posting an RFC and obtaining feedback about the proposal.
+
+Process
+=======
+
+Writing an RFC
+--------------
+The process begins with writing a proposal for the changes you'd like to see
+made. The proposal should include:
+
+* a high-level overview of what changes are being proposed,
+* information detailing the motivation for why the changes are being proposed,
+* detailed information the proposed changes and how they impact different parts
----------------
AaronBallman wrote:

Done

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