Add releasing steps

RELEASING.md

+Releasing
+========
+
+ 1. Change the version in `gradle.properties` to a non-SNAPSHOT verson.
+ 2. Update the `CHANGELOG.md` for the impending release.
+ 3. Update the `README.md` with the new version.
+ 4. `git commit -am "Prepare for release X.Y.Z."` (where X.Y.Z is the new version)
+ 5. `./gradlew clean uploadArchives`.
+ 6. Visit [Sonatype Nexus](https://oss.sonatype.org/) and promote the artifact.
+ 7. `git tag -a X.Y.X -m "Version X.Y.Z"` (where X.Y.Z is the new version)
+ 8. Update the `gradle.properties` to the next SNAPSHOT version.
+ 9. `git commit -am "Prepare next development version."`
+ 10. `git push && git push --tags`
+
+If step 5 or 6 fails, drop the Sonatype repo, fix the problem, commit, and start again at step 5.