Add option to output release-specific notes to `RELEASE_CHANGELOG.md` or an alternative file

[frankdugan3]

The motivation for this is that some tools (like GitHub releases) will not parse the message content of git tags as Markdown. GitHub releases does however support Markdown in the release body if you specify it manually, which can be done in CI.

So, to make that content available to other tooling, it would be ideal to optionally allow a separate file, such as RELEASE_CHANGELOG.md, which could be loaded directly