This library is using sbt-release-early for releasing artifacts. Every push will be released to maven central, see the plugins documentation on the versioning schema.
Choose the appropriate version number according to semver then create and push a tag with it, prefixed with v.
For example:
$ git tag -s v0.3.1
$ git push --tag
After pushing the tag, while it is not strictly necessary, please draft a release on github with this tag too.