Create a user guide

Using A/B Street is sometimes hard, especially for tools/features not covered in the tutorial. This issue is a duplicate of https://github.com/a-b-street/docs/issues/1, but in this repo just so I can tag it as "good first issue."

If anybody wants to help with this, docs in any format would help -- editing markdown in https://github.com/a-b-street/docs or even just a Google doc with pictures.