Cover forking workflow in contribution guidelines

At the moment the document only talks about feature branches on the canonical repo. This is appropriate for a private repo, but should be revisited whenever there's a public version.

We should rephrase to cover both cases *and* include a section in the developers' knowledge base. These figures might come in handy: https://psicode.org/psi4manual/master/build_obtaining.html#faq-forkpsi4public