[PATCH 3/2] update-index: list supported idx versions and their features

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 .. and the user should know (briefly) the differences between index
 versions too.

 Documentation/git-update-index.txt | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index 77a912d..e5aaba5 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -145,7 +145,15 @@ you will need to handle the situation manually.
 
 --index-version <n>::
 	Write the resulting index out in the named on-disk format version.
-	The current default version is 2.
+	Supported versions are 2, 3 and 4. The current default version is 2
+	or 3, depending on whether extra features are used, such as
+	`git add -N`.
++
+	Version 4 performs a simple pathname compression that could
+	reduce index size by 30%-50% on large repositories, which
+	results in faster load time. Version 4 is relatively young
+	(first released in 1.8.0 in October 2012). Other Git
+	implementations may not support it yet.
 
 -z::
 	Only meaningful with `--stdin` or `--index-info`; paths are
-- 
1.8.1.2.536.gf441e6d