allow adding release notes to PR body

[balvig]

Adding something like

_Release note: New search feature, please try it out_

to a PR's body, will use that instead of the title when composing release notes.
Could maybe skip the composing step if this turns out to be useful?

[Knack]

@balvig Nice! So, if I want to mark a PR as "internal" to hide it in release notes... maybe writing it as a comment will work? Something like:

_Release note: # this is an internal refactoring of some class_

[balvig]

@Knack hmm yes, or maybe in the future we leave out release notes for internal stuff? (and ignore PRs without release notes when posting to Slack) 🤔