Update doc to point out how to run a WAR as a JAR

[ChristerF]

From Ryan Kennedy: One thing that I haven't had a chance to point out in a pull request for the docs…the README mentions to either "Add instructions to also build a WAR" or "Change packaging of your Dropwizard service". You should be able to change the packaging to WAR and still invoke the WAR file like you would a JAR file as long as you update the build process to inject the missing Main-Class entry in the manifest (so you could java -jar application.war server application.yml) since the main class is still present in the WAR file and a WAR is just a JAR with a different suffix.