add docs on synccompress new args

docs/usage.rst

@@ -21,7 +21,7 @@ Management command
 You can update and force updates of the compressed file(s) with the management command “synccompress”.
 This makes it possible to keep the files updated manually.
 
-The command is (assuming you are in you project-folder that contains ``manage.py``). ::
+The command is (assuming you are in you project-folder that contains ``manage.py``) ::
 
     ./manage.py synccompress
 
@@ -29,6 +29,10 @@ To force all files to be re-generated, use the argument ``--force`` ::
   
     ./manage.py synccompress --force
 
+To re-generate only a specific group ::
+
+	./manage.py synccompress screen
+
 Templatetags
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