Add documentation on filter and populate usage

As mentioned in #72 I was going to send you the docs I wrote for the API I made with your module,
I've added them to the readme file. Let me know if you want anything changed
Thing is, i found that documentation on regexes and tried it but never saw it work every regex filter request gives me an empty array.

`/users?firstName__regex=/^sander/i` should give me 2 users but I get none
Same for `/users?gender__regex=/^male/i` should give me more than none.