Add resource overview table to docs about astroquery

[keflavich]

This should be something comparable to the "Currently Registered Packages" table on http://affiliated.astropy.org/?

[cdeil]

I don't really know any more what I meant at the time.

At the moment there is a simple list of abbreviations here:
http://astroquery.readthedocs.org/en/latest/

Maybe we could add a table or enumeration with some more information about what each service is, i.e. which waveband, catalogs or images, what the abbreviation stands for and a link to their website where people can find out more about it.

Many users that see astroquery for the first time don't know what is out there and what could be of interest for them.

Whether we best do this as an RST document or a JSON registry like the affiliated packages that can be queried I don't know.

[cdeil]

@keflavich Can you please fill out this list?
https://github.com/astropy/astroquery/wiki/Maintainers

For now I put it in the wiki, later we might want to list the maintainer in the overview table in the docs.