Add a changelog section to README

[timabbott]

This will be useful for documenting the API migrations (#6).

[ggrossetie]

It's probably better to use a dedicated file (i.e. CHANGELOG.md) with a link on the README? What do you think?

[ggrossetie]

Alternatively, we could describe changes in the release note since we have to create a new release in order to publish to https://github.com/marketplace?type=actions

[timabbott]

I think the "releases" tab approach can work OK, but it's often better to have a separate file, because then you can add notes on what is in master but not released. But it doesn't super matter; I think what's most important is that the README.md have a link to the changelog so that it's easy to find where we put it.