Add a note referencing media grouping earlier in the usage docs

Thanks to ksamuel for the original patch

docs/usage.txt

@@ -46,6 +46,10 @@ Which would be rendered something like:
 
     <script type="text/javascript" src="/static/CACHE/js/3f33b9146e12.js" charset="utf-8"></script>
 
+.. note::
+
+    Remember that django-compressor will try to :ref:`group ouputs by media <css_notes>`.
+
 Linked files **must** be accessible via
 :attr:`~django.conf.settings.COMPRESS_URL`.
 
@@ -204,6 +208,8 @@ sends the following arguments:
     every different media attribute on the tags within the ``{% compress %}``
     tag in question.
 
+.. _css_notes:
+
 CSS Notes
 ---------