README: Git is released under the GPLv2, not just "the GPL"

And this is clearly stressed by Linus in the COPYING file.  So make it
clear in the README as well, to avoid possible misunderstandings.
Signed-off-by: NStefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

README

@@ -19,7 +19,9 @@ Git is a fast, scalable, distributed revision control system with an
 unusually rich command set that provides both high-level operations
 and full access to internals.
 
-Git is an Open Source project covered by the GNU General Public License.
+Git is an Open Source project covered by the GNU General Public
+License version 2 (some parts of it are under different licenses,
+compatible with the GPLv2).
 It was originally written by Linus Torvalds with help of a group of
 hackers around the net. It is currently maintained by Junio C Hamano.