Add link to the changelog from the documentation.

[Mariatta]

I was reading the pyjwt documentation at https://pyjwt.readthedocs.io/en/stable/index.html and was looking for changelog of things that were changed between versions. I could not find such info in the doc, so I went to the source code on GitHub and then found the Chaneglog.md file.

I think it would be great if the Changelog is included as part of the documentation, or at least provide the link to it from the documentation. I can work on a PR for adding that to the doc if this is preferred.

Thanks.