Clean up README based on @nextmat comments in #165.

 - poor explanation of metrics definition - if the README is going to include copy/paste for everything else why not that?
 - for storage current instructions are easily read to suggest that you need both redis/mongo & a relational DB. I don't believe this is the case? I think most people reading the instructions would generate the migration and run it.
 - would be good to link to the identity section of the deeper docs in the README section about identifying users