Add example global config to Readme and wiki pages

It's very hard to find how to configure Timestamper though a configuration file. Just like in previous versions of the plugin it is explained how to enable the timestamps (e.g. with a timestamps {} block) there should be an example of how to set the global configuration.

Information such as, it needs to be an xml which roughly looks like the example below, should be easily accessible.

<?xml version='1.0' encoding='UTF-8'?>
<hudson.plugins.timestamper.TimestamperConfig plugin="timestamper@1.10">
    <timestampFormat>'<b>'HH:mm:ss'</b> '</timestampFormat>
    <elapsedTimeFormat>'<b>'HH:mm:ss.S'</b> '</elapsedTimeFormat>
    <allPipelines>true</allPipelines>
</hudson.plugins.timestamper.TimestamperConfig>

Please add this info to the Readme and to the wiki ([here|https://github.com/jenkinsci/timestamper-plugin] and [here|https://plugins.jenkins.io/timestamper] and [here|https://wiki.jenkins.io/display/JENKINS/Timestamper]