Add and keep a proper change log

[tysongach]

We’ve gotten serious about doing this with Bourbon and I think it would be quite valuable to do the same for Bitters.

• Here’s Bourbon’s change log
• Keep a CHANGELOG

[tysongach]

Also, I find SemVer a bit hazy when versioning UI, but this article has some particularly interesting thoughts on that: https://medium.com/design-optimizely/how-to-version-your-ui-library-1c7a1b7ee23a

Whatever way we handle it, we should document it.