Document git-gui, git-citool as mainporcelain manual pages

Document git-gui, git-citool as mainporcelain manual pages

Jakub Narebski pointed out that the git-gui blame viewer is not a
widely known feature, but is incredibly useful.  Part of the issue
is advertising.  Up until now we haven't even referenced git-gui from
within the core Git manual pages, mostly because I just wasn't sure
how I wanted to supply git-gui documentation to end-users, or how
that documentation should integrate with the core Git documentation.

Based upon Jakub's comment that many users may not even know that
the gui is available in a stock Git distribution I'm offering up
two basic manual pages: git-citool and git-gui.  These should offer
enough of a starting point for users to identify that the gui exists,
and how to start it.  Future releases of git-gui may contain their
own documentation system available from within a running git-gui.
But not today.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>