Add simple usage snippet to README

[tugberkugurlu]

Just like on https://github.com/tugberkugurlu/AspNet.Identity.RavenDB/blob/master/README.md

[dealproc]

Ok, with this mention, this is leading me to believe that the AddIdentity() methods should be within the main framework, and not in the example code. I hate to move it because it adds dependencies into the main framework, so maybe a good discussion to be had @tugberkugurlu to determine if an aux class should be established to handle any of the aspnet core specific bits vs the "netstandard" stuff that is currently housed in the main library.