Fix links to types of sitemaps in README

Under Features in the README, Video and News Sitemaps were broken links. I am new to Sitemaps, but I changed the links to Google pages that are probably the best resource.

It appears that Google no longer supports Geo Sitemaps, but since sitemap_generator still includes Geo Sitemaps as a feature, I just stubbed out the link, so someone else can decide where it should go.

I hope this PR is helpful.

-Mike