Add a specification for invitation URLs

[sgwilym]

earthstar-project/earthstar#36 describes an invitation URL format which has now received a second iteration. This invitation URL format makes it possible to produce single codes which can be exchanged between users and parsed by Earthstar applications, and there is an open PR for adding utilities for these codes to Earthstar's main API (earthstar-project/earthstar#313).

There should be a specification for creating and parsing these codes on the website!