Preview release notes when changesets are added to the PR #88

Hey folks!
Thanks for this great package and GitHub bot, we use it in a bunch of our projects, and it greatly simplifies our release pipeline.

I wanted to discuss adding a new feature to this bot. Currently, when you add a new changeset to the PR, following comment is added to it:

I believe it would be also useful to include release notes that will be generated using these changesets, as it takes place in PR that is created after running changeset version:

Of course, it doesn't have to be mandatory. There can be a flag that would determine whether this preview should be generated or not.

What do you think? If you accept external contributions, I'm more than happy to implement this, but I wanted to start the discussion first 🙂

Cheers!