README: Fix description of version numbering scheme

The version numbering scheme was changed in Oct, 2008.
This patch brings the documentation to the actual level.
The description is taken from:
http://www.denx.de/wiki/U-Boot/ReleaseCycle



Signed-off-by: Thomas Weber <weber@corscience.de>

Changed text slightly.
Signed-off-by: Wolfgang Denk <wd@denx.de>