Added affiliated package release instructions

As discussed in https://github.com/astropy/package-template/issues/103
@cdeil - just to be clear, do you mean specifically for the CHANGES file? I've recently taken to using ``README.md`` because I find it easier to write markdown than rst (e.g. links and images). GitHub creates README.md by default. Astropy core uses markdown for the contributing file (and even scikit-image uses readme.md for the readme file). So it's not clear to me that we should switch everything to rst. I feel like it would be an unecessary rule for which the benefits aren't really clear.

I agree with @eteq this needs rephrasing though.