docs(README.md): add command for cloning submodules to readme

Author: HacDan

README.md

@@ -100,6 +100,7 @@ Just create an empty directory and run the tests until you get a good result.
 ```sh
 git clone git@github.com:webinstall/webi-installers.git
 pushd ./webi-installers/
+git submodule update --init
 npm install
 ```