Add information about how to upgrade

### Description

The current README file doesn't tell users how to upgrade to the latest version of `xml2rfc`.
We should include a section about upgrading with `pip` and `pipx` (see #1042).

### Code of Conduct

- [X] I agree to follow the [IETF's Code of Conduct](https://github.com/ietf-tools/.github/blob/main/CODE_OF_CONDUCT.md)