doc(README.md): use 'npm clean-install'

Author: coolaj86

README.md

@@ -101,7 +101,7 @@ Just create an empty directory and run the tests until you get a good result.
 git clone git@github.com:webinstall/webi-installers.git
 pushd ./webi-installers/
 git submodule update --init
-npm install
+npm clean-install
 ```
 
 ```sh