Add v1.2 release notes. (opea-project#277)

* Add a preview of v1.2 release notes.

Signed-off-by: Yi Yao <[email protected]>
Co-authored-by: Dina Suehiro Jones <[email protected]>
Co-authored-by: Tian, Feng <[email protected]>
Co-authored-by: chen, suyue <[email protected]>
Signed-off-by: Pratool Bharti <[email protected]>