doc: reset release notes after 0.19 split-off
doc: reset release notes after 0.19 split-off

- empty release-notes.md and propose a few improvements

- delete release notes fragments


Former-commit-id: 2205fc6d68f87b817f104109703333f650a17de3
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin