Add release notes for 0.15.1
Add release notes for 0.15.1

Summary: Releade Notes

Test Plan: Check them by reading

Reviewers: deadalnix, kyuupichan, schancel, freetrader, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D586

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