Document core.excludesfile for git-add

During the discussion of core.excludesfile in the user-manual, I realized
that the configuration wasn't mentioned in the man pages.

Signed-off-by: Michael Hendricks <michael@ndrix.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-add.txt

@@ -62,6 +62,15 @@ OPTIONS
 	for command-line options).
 
 
+Configuration
+-------------
+
+The optional configuration variable 'core.excludesfile' indicates a path to a
+file containing patterns of file names to exclude from git-add, similar to
+$GIT_DIR/info/exclude.  Patterns in the exclude file are used in addition to
+those in info/exclude.  See link:repository-layout.html[repository layout].
+
+
 EXAMPLES
 --------
 git-add Documentation/\\*.txt::