Document Python API status

Closes #221

Author: jeromekelleher

docs/intro.md

@@ -23,5 +23,13 @@ support for other formats (or an interested in helping with implementing),
 please open an [issue on Github](https://github.com/sgkit-dev/bio2zarr/issues)
 to discuss!
 
+
 The package is currently focused on command line interfaces, but a
 Python API is also planned.
+
+:::{warning}
+Although it is possible to import the bio2zarr Python package
+the APIs are purely internal for the moment and will change
+in arbitrary ways. Please don't use them (or open issues about
+them on GitHub).
+:::