doc: document that labels must have a dot in them to have an effect
I noticed that the `hg topics` template has a bare `topic` label with
no dot, and that makes it useless, as such a label will never receive
any effect by the colour extension.
This dot has been required for a long time, at least since 2011, but
we never formally documented it!
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).