I've split this into two sections: submitting issues and contributing code, since those are two different actions.
This fixes#615 by mentioning that both CSS and SCSS source files should be updated when submitting a PR.
The referenced items are decisions we made for `_s`, that a re likely to be brought up again in pull requests or issues. Since our stance on them is not likely to change any time soon, let's include them in the Contributing FAQs.
See #464.
This might change in the future, based on our experiences, but should
be a good first draft of helpful and friendly guidelines for
contributing.
Props @bungeshea.
Fixes#381.