Added reference to contrib folder

doc/dict_legacy_xml.conf.sample

@@ -7,6 +7,8 @@
 # Note that this format, although more widely used, is less efficient than the
 # internal freeDiameter format. It is recommended when possible to use the later.
 
+# You may refer to the contrib/dict_legacy/README file for information on where to find such resources.
+
 # This file simply consists in a list of XML dictionary files that must be parsed.
 # Example:
 #  "/etc/freeDiameter/dictionary.xml";