Prepare 0.3.0 release (#48)

## What?

Prepares the 0.3.0 crate release.

## Why?

This will publish changes that were only partially available in the beta
release.

Merge pull request #27 from slowli/prepare-release

Prepare 0.3.0-beta.1 release

Bump crate version to 0.2.1

Merge pull request #15 from slowli/prepare-release

Prepare 0.2.0 release

Fix crate docs