Add release instructions

RELEASING.md

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+# Release
+
+Uses [axion release](https://axion-release-plugin.readthedocs.io/en/latest/) plugin.
+
+    ./gradlew release
+
+If you need to release with a SNAPSHOT dependency:
+
+    ./gradlew release -Prelease.disableChecks
+
+Otherwise, axion-release will fail the prerelease check. Only do this for testing.
+
+## Next minor version
+
+    ./gradlew markNextVersion -Prelease.incrementer=incrementMinor
+
+Updates minor version without releasing, e.g. 0.1.x-SNAPSHOT becomes 0.2.0-SNAPSHOT
+
+## Force version
+
+    ./gradlew release -Prelease.forceVersion=3.0.0