Set readme doc link based on phpBB versions

Author: iMattPro

skeleton/README.md.twig

@@ -11,7 +11,7 @@ This is a basic skeleton to help you start building a phpBB extension. Customise
 ## Extension Development Guide
 
 New to phpBB extension development?
-Start here: [phpBB Extension Development Guide](https://area51.phpbb.com/docs/dev/3.3.x/extensions/index.html)
+Start here: [phpBB Extension Development Guide](https://area51.phpbb.com/docs/dev/{{ skeleton_version_compare(REQUIREMENTS.phpbb_version_min, "4.0", "<") ? '3.3.x' : 'master' }}/extensions/index.html)
 
 ## License