README-publish

To publish:

- ./gradlew bintrayUpload will upload the release to JCenter. It will fail if the version is a SNAPSHOT
- ./gradlew uploadArchives will upload
  - the snapshot to https://oss.sonatype.org/content/repositories/snapshots
  - the release to https://oss.sonatype.org/service/local/staging/deploy/maven2
For a release, you then need to go to https://oss.sonatype.org/index.html#stagingRepositories to manually close and release the distribution.

Note that `./gradlew uploadArchives` is run by Travis after each new push to deploy the latest HEAD to the snapshot directory. As such, the version number of `build.gradle` in the git repo should always be -SNAPSHOT.