DOC: Add compatibility workflow badge

[rrjbca]

Description

An incompatibility with the astropy development branch was recently caught by our compatibility workflow but the failure was not noticed or acted on by developers before it was released. This pull request adds a status badge for the compatibility workflow to the README to increase the visibility of failures for developers.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[Lucia-Fonseca]

The badge is to increase the visibility of failures for developers but it will also be visible for the public. Do we really want that? Or it is not important?

[rrjbca]

The results of the compatibility workflow are already public: https://github.com/skypyproject/skypy/actions/workflows/compatibility.yaml