[llvm] [InstCombine] Add contributor guide (PR #79007)

[via llvm-commits]

goldsteinn wrote:

One more thing that might be nice is an example or so of successful PRs new contributors can look at to get a sense of how all these rules play out.
Think: https://github.com/llvm/llvm-project/pull/82929 and/or https://github.com/llvm/llvm-project/pull/78395 are pretty easy to follow and checks all the boxes.

Either way though, I think at this point the document is clearly useful and would be happy with it going up in its current form. We can always add more later.

https://github.com/llvm/llvm-project/pull/79007