Add resourceproxy extension to data viewer docs

@domoritz You said you would do this :)

Could we add a section to this page <http://docs.ckan.org/en/ckan-2.0/data-viewer.html> for the resourceproxy extension just like we have for each of the other extensions, mention what it does, how to enable it, and maybe also how extensions can use it (for someone writing a data viewer extension)