Create Uninstall Message hook

[kunikla]

Sample from DublinCoreExtended plugin
https://github.com/omeka/plugin-DublinCoreExtended/blob/master/DublinCoreExtendedPlugin.php

    public function hookUninstallMessage()
    {
        echo __('%sWarning%s: This will remove all the Dublin Core elements added '
        . 'by this plugin and permanently delete all element texts entered in those '
        . 'fields.%s', '<p><strong>', '</strong>', '</p>');
    }