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
This is used for filtering stringy fields. i.e. if you wanted to get a list of usernames that match a particular pattern

`/users?username__regex=/^baugarten/i` would search for all users whose username starts with baugarten (case-insensitive)

Follows the same rules as http://docs.mongodb.org/manual/reference/operator/query/regex/