Create RELEASES.md

[hdoupe]

This PR creates a RELEASES.md document modeled off of the Tax-Calculator RELEASES.md. Some work still needs to be done.

• 1. Sort the PR's that are currently listed in this document into API Changes, New Features, and Bug Fixes.
• 2. Add a highlights section that includes general user highlights and technical highlights. This section will be filled in by PolicyBrain developers as well as upstream package developers.

@martinholmer proposed the following process for keeping this document up to date.

When your pull requests are merged into the master branch, edit the top of the RELEASES.md file to include your merged contribution. After editing the file be sure to submit the revised file as a pull request (or you can do that edit as part of your substantive pull request).

I think that we should follow this here, especially since this process has been effective for Tax-Calculator.

[martinholmer]

@hdoupe, Having a RELEASES.md document for PolicyBrain will be very helpful. Thanks.

[MattHJensen]

+1. I may tinker with the highlights section in a follow up PR, but nonetheless lgtm.

[hdoupe]

@MattHJensen Great, thanks. Let me classify the the PR's as bug fixes, minor features, and major features. Then, I'll merge this.