Add sitemap.xml

On v7 I'm a big fan of https://github.com/marceldigital/Umbraco-XML-Sitemap but seems a v8 version isn't available yet...

So maybe https://github.com/callumbwhyte/friendly-sitemap ... seems it exludes pages that don't have a template... so for this site that should be sufficient... 