Add RELEASING.md (#338)

RELEASING.md

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+# Releasing
+
+## Releasing
+
+1. Update the version number in the `akka-grpc-xx-stable` project name in the [whitesource web UI](https://saas.whitesourcesoftware.com)
+    - For example you'd call the project `akka-grpc-0.2-stable`
+1. Create a [new release](https://github.com/akka/akka-grpc/releases/new) with the next tag version (e.g. `v0.2`), title and release description including notable changes mentioning external contributors.
+1. Travis CI will start a [CI build](https://travis-ci.org/akka/akka-grpc/builds) for the new tag and publish:
+  - SBT Plugin is published to [Bintray](https://bintray.com/akka/sbt-plugin-releases) that is linked the SBT plugins repo (no further steps required) 
+  - Gradle plugin directly to the Gradle plugin portal (no further steps required)
+  - Library jars to [Bintray](https://bintray.com/akka/maven) that needs to be synced with Maven Central
+1. Login to [Bintray](https://bintray.com/akka/maven/akka-grpc) and sync to Maven Central.
+1. Update quick start guides with the new version:
+  - https://github.com/akka/akka-grpc-quickstart-java.g8
+  - https://github.com/akka/akka-grpc-quickstart-scala.g8
+
+
+## Gradle plugin release details
+
+The Gradle plugin goes directly to the Gradle Plugin Portal. An encrypted `gradle.properties` that includes a
+publishing key and password is checked in under `gradle.properties.enc` and is decrypted by a private key known
+only to [travis](https://docs.travis-ci.com/user/encrypting-files/).