create task/build step to create sitemap.xml

[vincelee888]

to improve search engine optimisation, create a sitemap for crawling robots.

the site is built using https://metalsmith.io/. keep in mind a number of pages are created as part of the build process - take a look in tasks/ and any files called generate...

there's probably a plugin for metalsmith that will crawl its own files and generate one, otherwise, create a gulp task that can be run post-build that builds it!