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 think you should leave in this paragraph or else re-write this to add a "quick overview".  Now that we have space, the README page can have a paragraph describing the purpose of this repo for people that just wander by.