Added a mention of the need for a authz backend module (ie mod_authz_user)

<section id="example">

<title>Configuration Example</title>

<p>This simple example shows use of this module in the context of

the Authentication and DBD frameworks.</p>

the Authentication and DBD frameworks. Please note that you need

to load an authorization module, such as <module>mod_authz_user</module>,

to get it working.</p>

<example><pre>

# mod_dbd configuration

DBDriver pgsql