Add compatibility workflow badge (#487)

Author: rrjbca

README.rst

@@ -2,7 +2,7 @@
 SkyPy: A package for modelling the Universe
 ===========================================
 
-|Read the Docs| |GitHub| |Codecov|
+|Read the Docs| |GitHub| |Codecov| |Compatibility|
 
 This package contains methods for modelling the Universe, galaxies and the
 Milky Way. SkyPy simulates populations of astronomical objects, generating
@@ -68,7 +68,7 @@ You are welcome to talk about the SkyPy package and code using our
 `Discussions Page`_. For any other questions about the project in general,
 please get in touch with the `SkyPy Co-ordinators`_.
 
- .. _Discussions Page: https://github.com/skypyproject/skypy/discussions 
+ .. _Discussions Page: https://github.com/skypyproject/skypy/discussions
  .. _SkyPy Co-ordinators: mailto:skypy-coordinators@googlegroups.com
 
 Contributing
@@ -95,6 +95,9 @@ in the `Code of Conduct`_.
 .. |GitHub| image:: https://github.com/skypyproject/skypy/workflows/Tests/badge.svg
  :target: https://github.com/skypyproject/skypy/actions
 
+.. |Compatibility| image:: https://github.com/skypyproject/skypy/actions/workflows/compatibility.yaml/badge.svg
+ :target: https://github.com/skypyproject/skypy/actions/workflows/compatibility.yaml
+
 .. |Codecov| image:: https://codecov.io/gh/skypyproject/skypy/branch/main/graph/badge.svg
  :target: https://codecov.io/gh/skypyproject/skypy