add link to wayback machine's archive of attacklab

README.md

@@ -3,8 +3,9 @@ NOTE -- Showdown on GitHub
 ==========================
 
 Please note that I, Corey, am not the author of Showdown. Rather, I found it
-some time back at <http://attacklab.net/showdown/> and wanted to see it
-available on GitHub.
+some time back at <http://attacklab.net/showdown/>
+(website removed, see: <http://wayback.archive.org/web/*/http://attacklab.net/showdown>)
+and wanted to see it available on GitHub.
 
 All credit and praise for authoring this library should go to John Fraser.