Add release documentation & update version (#303)

Author: sergehuber

RELEASE.md

@@ -0,0 +1,18 @@
+RELEASE DOCUMENTATION
+---------------------
+
+Here are the steps to produce a release of the project:
+
+1. Check the gradle.properties file to make sure that the version number has been updated
+2. In the Github release UI, create a new tag with the following naming convention vXX.X and make sure you generate the release notes. 
+3. Create the release using the create release button
+4. Wait for the build action to complete successfully
+5. Launch the "Publish" action manually and wait for it to complete
+6. Check at the following URL that the new release was deployed : 
+
+    https://repo1.maven.org/maven2/io/github/graphql-java/graphql-java-annotations/
+
+   It might take some time for the release to appear
+7. Update this document if anything was missing or wasn't clear   
+8. Once everything is properly deployed, update the gradle.properties file to the next planned version, either directly (if that is the only change), or in a PR if other changes need to be done at the same time.
+9. Announce the release on your favorite communication channels.

gradle.properties

@@ -5,4 +5,4 @@ org.gradle.daemon=true
 org.gradle.parallel=true
 org.gradle.jvmargs=-Dfile.encoding=UTF-8
 
-version = 21.2
+version = 21.5