Add a UserWarning about the CLI interface

src/rover/__init__.py

@@ -1 +1,11 @@
+import warnings
+import textwrap as text
+
 from .rover import Rover
+
+def main():
+    warnings.warn(text.dedent(
+        """
+        There is still no command line for Rover.
+        Use a Python shell to interact with Rover.
+        """))