Documentation for memberships API

Preview this here: https://github.com/3dna/api_docs/blob/memberships/doc/memberships_api.md

@aq1018 @mjuhasz 
Earlier with @arionhardison we agreed on adding the definition of the REST resource to the top of the page, like [here](http://nationbuilder.com/contacts_api) or [here](http://nationbuilder.com/donations_api). It would be good to have the resource defined for this endpoint as well.