contributing: add new file with a pointer to the wiki
This also includes what I consider to be the minimum set of steps
someone should be able to perform even if they can't run the
testsuite. Hopefully this will help new contributors know to at least
run the two checkers that find most things that (in my experience)
require manual cleanup.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).