release: update release notes for 0.3.1

I forgot to do this when I bumped the version number.

release: bump version to 0.3.0

There's been *a lot* of changes since 0.2.0 almost a year ago. With
the attention the project has gotten recently, I feel like I should
cut a new release and start keeping a changelog. So let's start by
bumping the version to 0.3.0.