Added link to extensions in "Hook. Extend." section.

docs/becomingbig.rst

@@ -25,8 +25,9 @@ The :ref:`api` docs are full of available overrides, hook points, and
 response objects.  Dig deeper on the APIs you use, and look for the
 customizations which are available out of the box in a Flask release.  Look for
 ways in which your project can be refactored into a collection of utilities and
-Flask extensions.  Explore the many extensions in the community, and look for
+Flask extensions.  Explore the many `extensions
-patterns to build your own extensions if you do not find the tools you need.
+<http://flask.pocoo.org/extensions/>` in the community, and look for patterns to
+build your own extensions if you do not find the tools you need.
 
 Subclass.
 ---------