Bump version to 1.3.1 (#570)

* Bump version to 1.3.1

* Add Release documentation

Author: pglombardo

Diff for: RELEASE.md

@@ -0,0 +1,11 @@
+# How to Release a New Version of the Java Client
+
+1. Bump the version number in [gradle.properties](https://github.com/hivemq/hivemq-mqtt-client/blob/master/gradle.properties#L1)
+2. Create a new Github release with a v0.0.0 style tag
+
+## Notes
+
+Java client releases are done through a Github Action in [publish.yml](https://github.com/hivemq/hivemq-mqtt-client/blob/master/.github/workflows/publish.yml) that is triggered by publishing a new release.
+
+The publish action publishes the Java client library to Maven using the version in `gradle.properties`.
+

Diff for: gradle.properties

@@ -1,5 +1,5 @@
-version=1.3.0
-prevVersion=1.2.2
+version=1.3.1
+prevVersion=1.3.0
 #
 # main dependencies
 #