Update README.md

Author: adorton-adobe

README.md

@@ -24,7 +24,7 @@ To clone `user-guide` to a separate directory:
 $ git clone -b user-guide https://github.com/adobe-apiplatform/user-sync.py.git user-sync.py-docs
 ```
 
-The theme is installed as a submodule, so after cloning, update it:
+The theme is installed as a submodule, so after cloning, you must update it or else the project won't build:
 
 ```
 $ git submodule update --init --recursive