Add github access token to app.json

Author: kyb3r

app.json

@@ -26,6 +26,10 @@
     "OWNERS": {
         "description": "Comma seperated user IDs of people that are allowed to use owner only commands. (eval and update)",
         "required": false
+    },
+    "GITHUB_ACCESS_TOKEN": {
+        "description": "Github personal access token to enable use of the update command.",
+        "required": false
     }
   }
-}
+}