RFC: Introducing an LLVM Community Code of Conduct

+1 from this lurker.

Other communities have proven repeatedly why such CoC's are crucial, including the need for specific common offenses to be spelled out in detail, so we don't really need to approach this as if we don't know if or why such things are helpful. Even if the LLVM community has never had problems, there is merit to explicitly stating (even if non-exhaustively) LLVM's values and the boundaries of acceptable behavior and explicitly repudiating the kinds of shenanigans that have happened elsewhere.