Add changelog generator and "How to Release" info

fixes #313 
This comment would be more useful if it explained what a changelog was, e.g.

```
"""Prints the changelog.

This is a markdown-formatted list of all merged pull requests,
with their descriptions and links back to github, e.g.
* Issue #[1](http://github.com/owner/repo/pull/19): `Pull Request Description`
"""
```