Add instructions for how to release the library.

RELEASING.md

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+# Release process
+
+1. Commit any remaining changes and push. Wait for Travis build to come back green.
+2. Create Git tag with next version number:
+    * for major changes (v1.0.0 to v2.0.0): `./gradlew reckonTagCreate -Preckon.scope=major -Preckon.stage=final`
+    * for minor changes (v1.0.0 to v1.1.0): `./gradlew reckonTagCreate -Preckon.scope=minor -Preckon.stage=final`
+    * for patches (v1.0.0 to v1.0.1): `./gradlew reckonTagCreate -Preckon.scope=patch -Preckon.stage=final`
+
+3. Push tag. Travis will automatically create GitHub release with binaries and push them to OSSRH
+   (which will push them to Maven Central).
+4. Go to GitHub and add release notes / changelog to release.