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>

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index c14d1e9..8ac4975 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -5,15 +5,21 @@
 --------------------
 
 In order to use branch-network plugin as GitWeb project viewer replacement
-simply add the following line to your existing Gerrit config under the
-GitWeb section (assuming plugin was copied as branch-network.jar):
+simply add the following lines to your existing Gerrit config under the
+GitWeb section (assuming the plugin was copied as branch-network.jar):
 
 ```
   type = custom
-  url = plugins/
-  project = branch-network/?p=${project}
+  url = plugins/branch-network/
+  project = ?p=${project}
+  branch = ?p=${project}
+  revision = ?p=${project}
+  filehistory = ?p=${project}
 ```
 
+Note that the `project, ``branch`, `revision` and `filehistory` settings
+must all be specified, otherwise Gerrit will disable the configuration.
+
 Usage in other GWT or JavaScript UI
 -----------------------------------