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
I will make it url encoded now (however it encodes itself in most situations. but yes in your rewrite you could go for a comma, as it is used in the `$in` filter (`gender__in=male,female`).