docs: add link to issue reference

closes #1273

Readme.md

@@ -1207,7 +1207,7 @@ connection.query({
   }
 });
 ```
-__WARNING: YOU MUST INVOKE the parser using one of these three field functions in your custom typeCast callback. They can only be called once.( see #539 for discussion)__
+__WARNING: YOU MUST INVOKE the parser using one of these three field functions in your custom typeCast callback. They can only be called once. (see [#539](https://github.com/felixge/node-mysql/issues/539) for discussion)__
 
 ```
 field.string()