git-read-tree: document -v option.

Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -50,6 +50,9 @@ OPTIONS
 	trees that are not directly related to the current
 	working tree status into a temporary index file.
 
+-v::
+	Show the progress of checking files out.
+
 --trivial::
 	Restrict three-way merge by `git-read-tree` to happen
 	only if there is no file-level merging required, instead