package/doom-wad: add an upstream URL to Config.in

Add an upstream URL to the help text in Config.in. This
addresses the 'Missing' URL status in the package stats
web page output.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/doom-wad/Config.in

@@ -7,3 +7,5 @@ config BR2_PACKAGE_DOOM_WAD
 
 	  The wad file will be placed in the /usr/share/games/doom
 	  directory.
+
+	  https://doomwiki.org/wiki/DOOM1.WAD