Update README

Add links to SMWar repo.

README.md

@@ -6,7 +6,7 @@ Generate [Super Mario War](http://supermariowar.supersanctuary.net/) worlds from
 
 - Node.js (preferably latest)
 - Yarn
-- Existing installation of Super Mario War
+- Existing installation of Super Mario War (obtain v.2.0 from [Super Mario War's GitHub page](https://github.com/mmatyas/supermariowar/releases).
 
 ## Setup