Release Process - clarify release notes step

Summary:
Old release notes should be copied to archive during the
version number incrementing step.

Test Plan: Proofread, and make sure it makes sense

Reviewers: schancel, jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2112