[llvm] [RFC][AMDGPU][Doc] Add developer guideline (PR #194425)

[Chinmay Deshpande via llvm-commits]

https://github.com/chinmaydd approved this pull request.

I like what we have now. 

Though, I think this can be an evolving document that would accept edits depending on quirks found during future AMDGPU-specific reviews.

Ideally, we would not be required to repeat guidelines that fall into the LLVM coding standards (especially those that are specific to C++ and such). If there are strong objections from others regarding this, we should consider pushing such changes to the LLVM Coding Standards first. IMO it shouldnt block this PR.

The only other concern I have is that of ensuring that the overlapping points here remain consistent with changes as per the broader LLVM guidelines. There is no way to programmatically enforce this, but we should be on the lookout.

https://github.com/llvm/llvm-project/pull/194425