Chapter on what a web accl is

doc/sphinx/tutorial/web_accelerator.rst

+.. _tutorial-web_accelerator:
+
+What is a web accelerator
+-------------------------
+
+
+The problem
+-----------
+
+Application servers  er slow. They do lots of different things and
+they can sometimes take seconds to complete a web page.
+
+The solution
+------------
+
+Enter Varnish. It keeps a copy of the web pages that pass through
+it. If it finds that it can reuse these later so the server doesn't
+have to regenerate these it speeds things up.
+
+
+