Document the `/ma delete-session-stats` command

This command did not make it into the README on the initial commit. It should be fairly self-explanatory, but we should still have a section for it for completeness. The command implementation (along with usage message and permission) can be found here:

https://github.com/garbagemule/MobArenaStats/blob/master/src/main/java/org/mobarena/stats/command/DeleteSessionStatsCommand.java