Updated contrib docs, mention "DO NOT MERGE" pull requests

These should be avoided because they undermine the process of peer review and
merging in github, we should strive to have zero open pull requests, as
opposed to treating it as a stage for work in progress. Intermediary code
review can happen in github forks instead.

Also remove some checklist items which were based on the Trac bugtracker, e.g.
its not longer possible to assign yourself to issues because of github's
limited permission abilities.