Added a small explanation and an excerpt from a configuraton file to
the dbfwfilter documentation. It demonstrates the use of both blacklist
and whitelist functionality in the same service.
The `allow` keyword can be used to substitute the `deny` keyword but this
was not documented. Also the fact that neither of them affect the actual
behavior of the filter was not very clearly stated.
When configured to log matching queries, the dbfwfilter will log all queries
that match a rule. The rule name, user name and the query itself are logged.
It is also possible to match all queries that do not match a rule. Only the user
name and query is logged in this mode.
