Add documentation around posts__in

[kadamwhite]

Re: #117, @brad-decker indicates around supporting array filters:

You also need to add post__in and post__not_in to query_vars on your wordpress install.

This should be addressed in the readme or API documentation.

[kadamwhite]

Other things to document in the same mode: filtering by post status

[kadamwhite]

Technically post__in has been superseded by include[] so this should document whitelisting specific query variables in a general fashion, possibly by linking to the official docs

[kadamwhite]

With the removal of filter from the 4.7 API and the addition of include() / exclude(), this is no longer relevant. Documentation around how to modify the accepted parameters of an endpoint should live at the REST API developer handbook on developer.wordpress.org, not in a client library.