Add release notes template

[RCheesley]

This PR adds a release notes template which will make it easier to format changelogs when we run a release.

It requires the following labels, so we'll need to get into the habit of labelling PRs like we do in mautic/mautic:

• bc-break
• refactoring
• security
• feature
• enhancement
• bug
• regression
• chore
• devops