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.

CONTRIBUTING

@@ -0,0 +1,13 b''
+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).