credentials exports.tiles: Burwell tiles paths required to run API

[cambro]

v2 introduced the exports.tiles object in v2/credentials.js. This is required to start the API locally, which makes having Burwell operational, configured and in place a requirement to run the API. After configuring credentials in v1 and v2 and running npm start the following error is returned:

Error: Unable to initialize "burwell" layer: "ENOENT: no such file or directory, open '../macrostrat-api/v2/path/to/burwell_large.xml'"

Either the dependency on Burwell being in place and operational should be explained and instructions in Burwell repo updated, or it should be optional to involve exports.tiles during npm start.