Add link to Simple-Cone-Search-Creator to VO docs

@pllim @mdboom Do you think adding this link and one-sentence description of [Simple-Cone-Search-Creator](https://github.com/tboch/Simple-Cone-Search-Creator) by @tboch to the [astropy.vo](http://docs.astropy.org/en/latest/vo/index.html) docs is useful?