Update README.md

README.md

-This is mtime - a library for handling time, dates, and events in numerical models.
+This is `mtime` - a model time management library. It aims to provide time,
+calendar, and event handling for model applications.
+
+The library is [`REUSE`](https://reuse.software)-compliant and published under
+the BSD 3-clause license.
+
+You can find more information in the
+[documentation](https://icon-libraries.gitlab-pages.dkrz.de/libmtime) and
+[contribution guidelines](/CONTRIBUTING.md).