Add link to String.format reference.

Author: fdb

node/reference/string/format_number.md

@@ -9,3 +9,7 @@ Represent a numeric value as a string in a specific way.
 
 * **Value**: The input value
 * **Format**: The format template
+
+## Notes
+
+The format number string uses Java's string formatting function. [A full reference is here](http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html#syntax).