Changelog: Add explanatory note that this file is no longer updated

Add an explanatory note to the top of Changelog pointing at the
wiki and git history for changelogs for more recent releases.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

Changelog

+This file documents changes for QEMU releases 0.12 and earlier.
+For changelog information for later releases, see
+http://wiki.qemu.org/ChangeLog or look at the git history for
+more detailed information.
+
+
 version 0.12.0:
 
   - Update to SeaBIOS 0.5.0