Update README for local build

Signed-off-by: Erin Schnabel <schnabel@us.ibm.com>

Author: Erin Schnabel

README.md

@@ -4,6 +4,12 @@
 
 See the mediator service [information page](https://gameontext.gitbooks.io/gameon-gitbook/content/microservices/mediator.html) in the Game On! Docs for more information on how to use this service.
 
+## Building
+
+To build this project:
+
+    ./gradlew build
+    docker build -t gameontext/gameon-mediator mediator-wlpcfg
 
 ## Contributing