git-diff.txt: Link to git-difftool

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -157,6 +157,10 @@ $ git diff -R                          <2>
 rewrites (very expensive).
 <2> Output diff in reverse.
 
+SEE ALSO
+--------
+linkgit:git-difftool[1]::
+	Show changes using common diff tools
 
 Author
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