Add mention bot for Robin McCorkell

.mention-bot

@@ -5,6 +5,10 @@
     {
       "name": "DeepDiver1975", 
       "files": ["apps/dav/**"] 
+    },
+    {
+      "name": "Xenopathic",
+      "files": ["apps/files_external/**"]
     }
   ],
   "userBlacklist": ["owncloud-bot", "scrutinizer-auto-fixer"]