Note about static files dirs paths on Windows.

Added it to the settings reference. We used to have it in comment form
in our old, bigger default project template settings.py file.

docs/ref/settings.txt

@@ -2599,6 +2599,9 @@ your additional files directory(ies) e.g.::
         "/opt/webfiles/common",
     )
 
+Note that these paths should use Unix-style forward slashes, even on Windows
+(e.g. ``"C:/Users/user/mysite/extra_static_content"``).
+
 Prefixes (optional)
 ~~~~~~~~~~~~~~~~~~~