Add a new top-level section (with underscores, _)

user-manual.txt underlines the page title with underscores (_):

  Git User's Manual (for version 1.5.3 or newer)
  ______________________________________________

The major sections are underlined with equals signs (=).  This is
causing the page to be truncated before the second major section
(Exploring Git history) [1].

With this commit, we should get the whole manual, but it might also
cause problems by demoting all other section headers (<h1> -> <h2>).
It *shouldn't* cause problems, since HTML5 only uses relative header
sizes for implicit sectioning [2], but that doesn't mean it won't ;).

[1]: http://git-scm.com/docs/user-manual.html
[2]: http://www.w3.org/html/wg/drafts/html/master/sections.html#headings-and-sections
Merge pull request #280 from wking/user-manual-truncation

user-manual.html truncated after the first section