Make explanatory documentation

[tomcontileslie]

So that other users can contribute more effectively, we should create some explanatory documents showing - perhaps in the form of a flowchart - the full process of handling a chord request.

This could include a description of the broad thatchord -> settings -> interpret -> find -> rank -> output workflow, as well as more in-depth discussion of the finder algorithm, which is perhaps the least self-explanatory at first glance.