Add intro text for Extending Wagtail section

docs/extending/index.rst

@@ -1,6 +1,10 @@
 Extending Wagtail
 =================
 
+The Wagtail admin interface is a suite of Django apps, and so the familiar concepts from Django development - views, templates, URL routes and so on - can be used to add new functionality to Wagtail. Numerous `third-party packages <https://wagtail.io/packages/>`_ can be installed to extend Wagtail's capabilities.
+
+This section describes the various mechanisms that can be used to integrate your own code into Wagtail's admin interface.
+
 
 .. toctree::
     :maxdepth: 2