Add development/affiliated-packages.rst docs page

We discussed in https://github.com/astropy/package-template/issues/96 where to put the `package-template` docs and there was agreement that a page in the Astropy docs would be a good idea.

At the moment this is a copy of the `package-template/README.rst` page,
but the idea is that this documentation is incrementally restructured and improved after moving it here.

cc @eteq @astrofrog @mwcraig