Add release notes

Summary:
Draft spec for the network upgrade is here: https://github.com/bitcoincashorg/bitcoincash.org/pull/509

Depends on D7200

Test Plan: Read it and verify network upgrade timestamp.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D7199