When rendered as a page, release notes are a little odd as there is no title.
(see https://www.bitcoinabc.org/doc/release-notes/release-notes-0.22.2.html for example)
This patch adds a heading to the release notes template and updates all past ABC
release notes using
cd doc/release-notes grep -o "ABC version [0-9.]*" ./*.md | cut -d' ' -f3 | \ xargs -n1 -I{} sed -i '1s/^/# Bitcoin ABC {} Release Notes\n\n/' ./release-notes-{}.md