Update github.md

doc/integration/github.md

@@ -19,7 +19,7 @@ GitHub will generate an application ID and secret key for you to use.
     - Application name: This can be anything. Consider something like `<Organization>'s GitLab` or `<Your Name>'s GitLab` or something else descriptive.
     - Homepage URL: The URL to your GitLab installation. 'https://gitlab.company.com'
     - Application description: Fill this in if you wish.
-    - Authorization callback URL is 'http(s)://${YOUR_DOMAIN}'
+    - Authorization callback URL is 'http(s)://${YOUR_DOMAIN}'. Please make sure the port is included if your Gitlab instance is not configured on default port.
 1.  Select "Register application".
 
 1.  You should now see a Client ID and Client Secret near the top right of the page (see screenshot).