Initial commit of CONTRIBUTING.md

Unfinished, working on it now.

/cc https://github.com/cdnjs/cdnjs/issues/3388
Typo
Policy tweaks to CONTRIBUTING.md

/cc https://github.com/cdnjs/cdnjs/issues/3388
More work on CONTRIBUTING.md

/cc https://github.com/cdnjs/cdnjs/issues/3388
Tweak CONTRIBUTING.md
More CONTRIBUTING tweaks

That’s all for now, more later.
Bump gist-embed version

/cc https://github.com/cdnjs/cdnjs/pull/3392