Add sitemap.xml

Add sitemap for the site.  Please note that we don't specify lastmod for
posts because some of the them use the year 0000...

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Add robots.txt

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #27 from penberg/robots

Make site more accessible for robots