Full Path on robots.txt for sitemap.xml, compatible with Google, Ask, Bing and Yahoo

[fititnt]

Same as webiwg/webiwg-issues#14

See https://developers.google.com/webmasters/control-crawl-index/docs/robots_txt?hl=pt-br#sitemap and http://www.sitemaps.org/protocol.html#informing

# robotstxt.org/

User-agent: *

If is not possible (or even a good idea do it by default) generate a robots.txt with full url, at least add to robots.txt one comment that make a protip of adding the URL of sitemap.