[docs] GitHub Getting Started Request for Review

Hey folks, another request for review is ready! Here is the start for a GitHub getting started guide. I suspect this needs a lot of love, and (in the future) will benefit from more screenshots / interactive content. For now, it’s just text. https://github.com/sourcecred/docs/pull/12

Note that if we reorganized the categories and tags, this would be category “Community” and tag “documentation.”