Added notes about CSS rewriting tasks

README.md

@@ -24,6 +24,8 @@ In your `environments/production.rb`:
     config.action_controller.asset_host = "http://cdn.url.gov.uk"
     config.action_controller.asset_path = CdnHelpers::AssetPath
 
+The gem also make the `rake` task `cdn:css_urls` available to you. This is for post-deployment rewriting of URLs within CSS files. It's destructive - it rewrites the files in place - but it doesn't require that the config vars above are set since it leaves URLs as absolute local URLs.
+
 Django
 ------
 
@@ -49,3 +51,5 @@ in templates use the `asset_url` helper:
 
     <img src="{% asset_url "/images/thing.png" %}">
     <img src="{% asset_url thing.image.src %}">
+
+The app provides the `cdn_css` command for `manage.py`. This uses the Ruby script process_css to do the heavy lifting, but relies on the `MEDIA_ROOT` and `MEDIA_URL` settings in `settings.py` to tell where to look and how to construct the URLs.