Readme.md: Add edk2 release tag and edk2 release plan
Readme.md: Add edk2 release tag and edk2 release plan

https://bugzilla.tianocore.org/show_bug.cgi?id=1364

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2