Update build instructions

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "extern/Universal-Image-Loader"]
 	path = extern/Universal-Image-Loader
 	url = https://github.com/nostra13/Android-Universal-Image-Loader
+    ignore = dirty

README.md

@@ -11,7 +11,9 @@ Building from source
 The only required tools are the Android SDK and Apache Ant.
 
 ```
+git submodule update --init
 android update project -p .
+android update project -p extern/Universal-Image-Loader/library
 ant clean release
 ```