docs: add explicit CalVer versioning section to README

- Introduced a "Versioning" section describing the Calendar Versioning (CalVer) scheme used by the project.
- Provided format details and an example for clarity.

Change-Id: I98eb1b564e5aec7666065241e3257d917adcd2f2
Signed-off-by: Peng Sun <peng.sun@arm.com>