Discussion: Clarify contributor guidelines

There was some discussion in dotnet/wpf#9981 about what kind of changes should be included in a pull-request (The main point discussed in dotnet/wpf#9981 was about unrelated changes for refactoring/formatting).

I took the liberty of opening this issue to discuss it with the community and the WPF team. Obviously, I think we are free to discuss it but the WPF team will have the final say about this.

The end goal of this issue is to document it properly and for the guidelines to be clear to set the right expectation for a PR.