Update release notes for 0.18.0

Summary: Add missing items to release notes

Test Plan: Read it over, and check that nothing significant you worked on is missing

Reviewers: deadalnix, schancel, jasonbcox, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1639