Add references to `astroquery` and `pyvo` in `astropy.vo` docs

We just added a reference from `astroquery` to `astropy.vo` and `pyvo` (see [here](http://astroquery.readthedocs.org/en/latest/#introduction)).

Would it make sense to add a few sentences at the beginning of the `astropy.vo` docs (see [here](http://docs.astropy.org/en/latest/vo/index.html)) mentioning `pyvo` and `astroquery`?

What to say really depends on whether we plan to merge `pyvo` and / or `astroquery` into `astropy` core soon (say for the 0.3 or 0.4 release) or whether we want them to remain affiliated packages.

If `pyvo` is not merged into core `astropy` soon it should also be listed on the [astropy affiliated package page](http://www.astropy.org/affiliated/).

@mdboom @RayPlante @keflavich What do you think?