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>

Author: Thomas Weber

README

@@ -126,13 +126,17 @@ the string "u_boot" or on "U_BOOT". Example:
 Versioning:
 ===========
 
-U-Boot uses a 3 level version number containing a version, a
-sub-version, and a patchlevel: "U-Boot-2.34.5" means version "2",
-sub-version "34", and patchlevel "4".
-
-The patchlevel is used to indicate certain stages of development
-between released versions, i. e. officially released versions of
-U-Boot will always have a patchlevel of "0".
+Starting with the release in October 2008, the names of the releases
+were changed from numerical release numbers without deeper meaning
+into a time stamp based numbering. Regular releases are identified by
+names consisting of the calendar year and month of the release date.
+Additional fields (if present) indicate release candidates or bug fix
+releases in "stable" maintenance trees.
+
+Examples:
+	U-Boot v2009.11     - Release November 2009
+	U-Boot v2009.11.1   - Release 1 in version November 2009 stable tree
+	U-Boot v2010.09-rc1 - Release candiate 1 for September 2010 release
 
 
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