Explain requirements to convert someone else's patch on b.p.o to a PR

There's a note [here](http://cpython-devguide.readthedocs.io/pullrequest.html#quick-guide-step-by-step) that says:
`You can still upload a patch to bugs.python.org, but the new pull request workflow is preferred.`

While patches can still be uploaded to b.p.o, in the end we still ask people to create a PR based on that patch anyway. Maybe this note should be removed?
