Update README with a current feature overview and MSRV

[AlexanderBand]

A reminder to update the README. The current list of features is outdated, as well as the minimum supported Rust versions. An alternative would be to take these items out and refer to the documentation?

Also, CONTRIBUTING.md was last updated a year ago is is currently descibed as a "first draft version". Review if it is still what we want/need.