Added more precision to the predefined URLs and titles feature.

PHP Markdown Readme.text

@@ -224,12 +224,20 @@ Current:
 	as a code block. This has been fixed.
 
 *	Added programatically-settable parser properties `predef_urls` and 
-	`predef_titles` for predefined URLs and link titles. To use this, 
-	your PHP code must call the parser this way:
+	`predef_titles` for predefined URLs and titles for reference-style 
+	links. To use this, your PHP code must call the parser this way:
 
 		$parser = new Markdwon_Parser;
-		$parser->predef_urls['my_url_ref'] = 'http://example.com';
+		$parser->predef_urls = array('linkref' => 'http://example.com');
 		$html = $parser->transform($text);
+
+	You can then use the URL as a normal link reference:
+
+		[my link][linkref]	
+		[my link][linkRef]
+
+	Reference names in the parser properties *must* be lowercase.
+	Reference names in the Markdown source may have any case.
 
 *	Added `setup` and `teardown` methods which can be used by subclassers
 	as hook points to arrange the state of some parser variables before and