title: Confusion about sitemap_index.xml
body: <p>I've read the Google docs on sitemap formats but there's one thing they don't make clear:
  will search engines automatically look for and find /sitemap_index.xml or do you have to tell them about it via /robots.txt or the main /sitemap.xml?  Can you <em>not</em> have a /sitemap.xml and still rely on /sitemap_index.xml to be found and harvested?</p>