add docs for filtering

[sanderhouttekier]

I've been looking trough your code and found out how you pass querystring parameters into the mongoose query to limit the numbers or populate subentities.

also found out you can pass any parameter from an entity to filter like
/users/?gender=male

but i also found your list of subfilters but have yet to figure out how they are used ...

it would be great if you could just hint in the right direction, on how to use them
then i'd be happy to send you a pull request with an updated readme.md file where the use of these filters is mentioned. With some examples or so.

[sanderhouttekier]

After debugging your code i found the usage for the filters, I am documenting them on a wiki page for our project and if you want that I can rework them into a pullrequest on this app so you have it in here too for future users.

[baugarten]

Yes, please. That would be wonderful

I'm mid rewrite, but the API for filters will stay the same