Create playbook

The project documentation should contain a playbook describing how to use the STAC Auth Proxy.

Some ideas:

- [ ] authenticated users can access transaction endpoints. Anonymous users have read-only access
- [ ] the collections in which a user is permitted to view is encoded as an array of IDs in the auth token
- [ ] the collections in which a user is permitted to view is returned from a DB lookup
- [ ] anonymous users cannot read items containing a `private: true` property. Authenticated users can read all records