Update README.

readme.md

@@ -56,7 +56,6 @@ Import the mixins at the beginning of your stylesheet
 # Install without Rails
 Bourbon includes an easy way to generate a directory with all the necessary files.
 
-    gem install bourbon
     bourbon generate
 
 This will create a `bourbon` directory that can be used by `sass --watch`. Move
@@ -71,6 +70,12 @@ In this case, you will need to import the mixins at the beginning of your styles
 
     @import 'bourbon/bourbon';
 
+To update your existing bourbon files, do this:
+
+    bourbon update
+
+This will delete the bourbon directory and regenerate it.
+
 # Browser support
 Bourbon aims to provide support for CSS3 properties that are not yet fully supported in modern stable browsers.