Add option variable for class variables

doc/python-syntax.txt

@@ -86,6 +86,9 @@ following command to your `~/.config/nvim/init.vim` or `~/.vimrc`: >
 `g:python_highlight_doctests` (default `0`)
     Highlight doc-tests
 
+`g:python_highlight_class_vars` (default `0`)
+    Highlight class variables `self` and `cls`
+
 `g:python_highlight_all` (default `0`)
     Enable all highlight options above, except for previously set.