Added affiliated package release instructions

As discussed in https://github.com/astropy/package-template/issues/103
In the spirit of "There should be one-- and preferably only one --obvious way to do it." from `import this`, I think it would be better to make a decision to use `.rst` format and to only add a note here that if your package currently uses the `.md` format you are encouraged to re-format to `rst`.

It's a bit painful for people that have `.md` now, but then it will be simpler for the coming decades.

Choosing `.rst` is better than `.md` because all Astropy and affiliated package docs are already `.rst`.