Add extensions section to Quickstart documentation

docs/quickstart.rst

@@ -873,6 +873,15 @@ can do it like this::
     from werkzeug.contrib.fixers import LighttpdCGIRootFix
     app.wsgi_app = LighttpdCGIRootFix(app.wsgi_app)
 
+Using Flask Extensions
+----------------------
+
+Extensions are packages that help you accomplish common tasks. For
+example, Flask-SQLAlchemy provides SQLAlchemy support that makes it simple
+and easy to use with Flask.
+
+For more on Flask extensions, have a look at :ref:`extensions`.
+
 Deploying to a Web Server
 -------------------------