Automate contributor list update in a major astropy release

As mentioned by @bsipocz . Currently, here are the relevant steps as laid out in http://docs.astropy.org/en/latest/development/releasing.html for a major `astropy` release:

1. Run the `git shortlog -n -s -e` to see the list of all contributors and their email addresses.
2. Look for any misnamed entries or duplicates.
3. Add them to the `.mailmap` file (matched to the appropriate canonical name/email address).
4. Count the number of lines in `git shortlog -s` to get the final contributor count.
5. Update the `docs/credits.rst` file to include any new contributors.
6. May also need to update the contributor list on the [web site](https://github.com/astropy/astropy.github.com) if you updated the `docs/credits.rst` at the outset.

These steps can be tedious and boring for a human, therefore, they should be automated where possible.

Brigitta mentioned running `xonsh generate_releaserst.xsh 3.1 --project-name=astropy` for one of the steps above but I am not sure where that script currently resides.

Start with the release procedure in docs/releasing.rst and inspect the referenced generate_releaserst.xsh command to locate the existing automation. Review how .mailmap and docs/credits.rst are updated during a major release, then define the automated workflow and verify that it produces the expected contributor list and count.

Written by the indexing model from the issue text.