X-UA-Compatible meta tag missing, sitemap, and robots.txt example

The X-UA-Compatible meta tag was missing from some HTML files.

robots.txt has an example for allowing all URLs to be crawled, but doesn't have an example for disallowing all URLs to be crawled.

An example sitemap was added. (Though I'm not sure if <loc>http://example.com/</loc> would fly with the Web Starter Kit guidelines.)
@sindresorhus I wasn't against adding it, I just wanted to clarify things.

Also, I don't know how much sense does it make to provide this page here: if the user doesn't configure its server to return this particular 404 page, the page won't really be used. In H5BP, the idea is that the server configs (Apache or the alternatives) will basically ensure that the user's server will return the `404.html` page in case of an 404 error.