Release Notes for Version changes

[tylerbarna]

Feature Summary
Now that we're more consistently putting out releases of nmma, it might be good to have more documentation regarding the changes between numbered releases

Usage / behavior
This wouldn't be particularly user-facing in regards to actually using the package, but release pages would be accompanied by release notes that detail what has changed since the previous numbered version.

Alternative Solutions
While these can be manually written, there also seems to be a GitHub tool for automatically generating basic release notes that could be used to reduce the time required to implement it.

Implementation details
Include a readme file in numbered releases (example: numpy) and/or maintain a running changelog that is linked to each numbered release (example: astropy)