user-manual: mention git-gui

The git gui project seems to be still in early stages, but at a point
where it's worth mentioning as an alternative way of creating commits.

One feature of interest is the ability to manipulate individual diff
hunks.  However, people have found that feature not to be easily
discoverable from the user-interface.  Pending some ui improvements, a
parenthetical hint here may help.

(Thanks to Steffen Prohask and Junio Hamano for suggesting the
language.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -1079,6 +1079,11 @@ $ git diff HEAD	    # difference between HEAD and working tree; what
 $ git status	    # a brief per-file summary of the above.
 -------------------------------------------------
 
+You can also use gitlink:git-gui[1] to create commits, view changes in
+the index and the working tree files, and individually select diff hunks
+for inclusion in the index (by right-clicking on the diff hunk and
+choosing "Stage Hunk For Commit").
+
 [[creating-good-commit-messages]]
 Creating good commit messages
 -----------------------------
@@ -2506,8 +2511,10 @@ $ gitk origin..mywork &
 
 And browse through the list of patches in the mywork branch using gitk,
 applying them (possibly in a different order) to mywork-new using
-cherry-pick, and possibly modifying them as you go using commit
---amend.
+cherry-pick, and possibly modifying them as you go using commit --amend.
+The git-gui[1] command may also help as it allows you to individually
+select diff hunks for inclusion in the index (by right-clicking on the
+diff hunk and choosing "Stage Hunk for Commit").
 
 Another technique is to use git-format-patch to create a series of
 patches, then reset the state to before the patches: