Document Mapbox setup

[abettermap]

Add steps to README so they can be recycled. Throw in some links, maybe a screenshot.

1. Create acct
2. Get API token
3. Add to .env file (commit sample.env) not worth it yet, only one var so far

[abettermap]

Future Jason: start with this doc and put into README. Also look at react-map-gl README to see if they have any extra info/advice.

[abettermap]

Just to comment on a concern that came up in an early discussion (not wanting to be "locked in" to a particular service)- vector tiles can be self-hosted. So, if for some reason there was a decision to jump the Mapbox ship then the tiles could conceivably live elsewhere.