Add link to Docker official image

Author: headius

README.md

@@ -5,3 +5,5 @@ Builds jruby docker images.
 
 To update the images, specify the version in `<major>/VERSION`. This should be
 the full version.
+
+To update the published images, send a pull request for https://github.com/docker-library/official-images/blob/master/library/jruby