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.

Signed-off-by: Peng Sun <peng.sun@arm.com>
Change-Id: Ia2a60bebf55676e29953d6d31cd8a6e5b1f2b222