Add install instruction to clear node_modules

This is needed to keep the user from getting a seg fault.
adopted-ember-addons/ember-cli-sass#117

README.md

@@ -9,8 +9,12 @@ A very simple shim to add [Materialize](https://github.com/Dogfalo/materialize)
 Install this addon into your ember.js app, using ember-cli
 ```sh
 ember install ember-materialize-shim
+rm -rf node_modules
+npm install
 ```
 
+You must clear your node_modules and reinstall dependencies.  This is to ensure you have an older version of node-sass which is required to circumvent an outstanding issue.  See: https://github.com/aexmachina/ember-cli-sass/issues/117
+
 And then you can import materialize styles
 
 **app/styles/app.scss**