Add tip for using environment variables

supervisor/skel/sample.conf

@@ -1,4 +1,10 @@
 ; Sample supervisor config file.
+;
+; For more information on the config file, please see:
+; http://supervisord.org/configuration.html
+;
+; Note: shell expansion ("~" or "$HOME") is not supported.  Environment
+; variables can be expanded using this syntax: "%(ENV_HOME)s".
 
 [unix_http_server]
 file=/tmp/supervisor.sock   ; (the path to the socket file)