Clarify contribution guidelines

As seen in #591 there is a little confusion when new contributors can't find contribution guidelines. https://github.com/dotnet/diagnostics/issues/591#issuecomment-578856530

We should make sure there is a clear link to this information on the front page of the repo. (We may also want to take this opportunity to organize the README a bit, a bunch of the details about SOS and lldb probably don't belong in the first screen of text)