Add css note

#572

README.md

@@ -538,7 +538,7 @@ Gollum optionally takes a `--config file`. See [config.rb](https://github.com/gi
 ## CUSTOM CSS
-The `--css` flag will inject `custom.css` from the root of your git repository into each page. Here's an example of floating the sidebar to the left.
+The `--css` flag will inject `custom.css` from the root of your git repository into each page. `custom.css` must be commited to git or you will get a 302 redirect to the create page. Here's an example of floating the sidebar to the left.
 ```css
 #wiki-rightbar { float: left !important; }