git-config: document interactive.singlekey requires Term::ReadKey

Most distributions don't require Term::ReadKey as dependency, leaving
the user to wonder why the setting doesn't work.

Signed-off-by: Simon Ruderich <simon@ruderich.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1629,7 +1629,7 @@ interactive.singlekey::
 	linkgit:git-add[1], linkgit:git-checkout[1], linkgit:git-commit[1],
 	linkgit:git-reset[1], and linkgit:git-stash[1]. Note that this
 	setting is silently ignored if portable keystroke input
-	is not available.
+	is not available; requires the Perl module Term::ReadKey.
 
 log.abbrevCommit::
 	If true, makes linkgit:git-log[1], linkgit:git-show[1], and