README.md in materialist-3.3.0 vs README.md in materialist-3.4.0

@@ -12,9 +12,12 @@
 
 In your `gemfile`
 
 ```ruby
 gem 'materialist'
+
+# or for sandbox features
+gem 'materialist', github: 'deliveroo/materialist', branch: 'sandbox'
 ```
 
 Then do
 
 ```bash