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.