Add usage documentation

[dhmay]

Is your feature request related to a problem? Please describe. jamstats usage is getting increasingly complex.

Describe the solution you'd like Make some usage documentation for the tool. Describe commandline arguments in detail, and give examples. Explain all the things you can do in the web ui (that could be deployed as online documentation by the server).

Additional context I don't really know who the potential users are, but the better the documentation the more of them there can be.