Added note for escaping percent signs

This is documentation for #291

docs/configuration.rst

@@ -378,10 +378,12 @@ follows.
   directory in which the supervisord configuration file was found.
   Values containing non-alphanumeric characters should be quoted
   (e.g. ``KEY="val:123",KEY2="val,456"``).  Otherwise, quoting the
-  values is optional but recommended.  **Note** that subprocesses will
-  inherit the environment variables of the shell used to start
-  :program:`supervisord` except for the ones overridden here and within
-  the program's ``environment`` option. See :ref:`subprocess_environment`.
+  values is optional but recommended.  To escape percent characters,
+  simply use two. (e.g. ``URI="/first%%20name"``) **Note** that 
+  subprocesses will inherit the environment variables of the shell 
+  used to start :program:`supervisord` except for the ones overridden
+  here and within the program's ``environment`` option.  See 
+  :ref:`subprocess_environment`.
 
   *Default*: no values