readline: add project URL

Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/readline/Config.in

@@ -5,3 +5,5 @@ config BR2_PACKAGE_READLINE
 	  The GNU Readline library provides a set of functions for use
 	  by applications that allow users to edit command lines
 	  as they are typed in.
+
+	  https://cnswww.cns.cwru.edu/php/chet/readline/rltop.html