Remove affiliated package instructions from README page

This pull request removes the instructions from the README page.

This has been discussed and I think agreed on in #96.
https://github.com/astropy/astropy/pull/2800 should be merged first.

cc @astrofrog @eteq @mwcraig
I much prefer a link to existing infos instead of duplicating them, but it's a matter of style and if you prefer to put a short description here, fine.

@eteq, if you don't mind adding the text yourself here, please go ahead and make a pull request against this one or a new one.