Improve documentation of plugin's configuration

Gerrit expects the `branch`, `revision` and `filehistory` settings
to be populated in the gitweb section of the configuration file.

Add them in the example in the documentation.

Change-Id: Ifc40d1070878e93b5d5c9b0ac90f0c452062a266
Signed-off-by: David Pursehouse <david.pursehouse@sonymobile.com>