Updated release notes to make it clear where the Github release contents come from
Updated release notes to make it clear where the Github release contents come from

Summary: see title

Test Plan: Did this myself

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1663

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