Added instructions on setting up affiliated packages manually

@cdeil - as discussed previously... this is overdue!

@Cadair - you may also want to review this.

Preview: http://www.mpia-hd.mpg.de/~robitaille/astropy/docs_affiliated/development/affiliated-packages.html

This includes a section on updating for the manual method, but still needs instructions for updating when using the git method. I can try and add that later. In the mean time, a review of what's here already would be good.

**Note 1:** this may still contain mistakes because I haven't gone through and tried it out, but it would be helpful if someone could try it out just to see if they end up with a functional package.

**Note 2:** I changed ``yourpkg`` to ``packagename`` to match the package template otherwise it's confusing.

Question: is it worth splitting the two main sections into sub-pages now that it's all quite long?
Did you edit the ``setup.cfg`` file as indicated?