add_stylesheet('railroad-diagrams.css') for syntax diagrams

After reading thru readthedocs/readthedocs.org#518
and experimenting, this seems to be the way to get the syntax diagram
style sheets right on readthedocs.org

Author: dckc

docs/source/conf.py

@@ -30,6 +30,9 @@
 # ones.
 extensions = ['rr_ext']
 
+def setup(app):
+    app.add_stylesheet('railroad-diagrams.css')
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']