Creating changelog and/or upgrade guide?

[cholmes]

@kylebarron suggested in #77 (comment) that we should consider adding a changelog. And perhaps even an 'upgrade guide'.

I'm generally into the idea. I think a changelog is pretty straightforward - with github's autogeneration of release notes I think we can just copy from that into a file. But having that as a single file in the repo is easier to find and check out than looking at each individual release note. So I may try to just make a PR for that.

Curious about what people think for an upgrade guide - what would need to go for it for a spec, if it's overkill, etc.

[cholmes]

Added #85 to discuss the changelog part of this.