Doc and metadata updates

[urschrei]

• [x] I agree to follow the project's code of conduct.

• [ ] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

This updates the top-level README to make key features a little more obvious, and updates the metadata with relevant keywords.

[adamreichold]

bors r+

[bors[bot]]

Build succeeded:

• ci result