Add a changelog

[asmeurer]

We should add a changelog that lists all the changes between versions. That will make it easier for implementers to see what they need to update in their implementations.

Additionally, we can use the versionadded and versionchanged Sphinx directives (https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-versionadded) to add a note to each respective function about things that have changed, which will make it easier to see what changed for a specific functionality.

[rgommers]

The changelog has been added in gh-574. The versionadded and versionchanged directives can be added based on that later - that'll be a task for after the v2022 release

[kgryte]

versionadded and versionchanged directives have been added to the specification. As such, will go ahead and close this one out.