Add Hubbard model description into __main__.py.

Author: hzhangxyz

qmb/__main__.py

@@ -7,6 +7,7 @@
 import tyro
 from . import openfermion as _  # type: ignore[no-redef]
 from . import fcidump as _  # type: ignore[no-redef]
+from . import hubbard as _  # type: ignore[no-redef]
 from . import ising as _  # type: ignore[no-redef]
 from . import vmc as _  # type: ignore[no-redef]
 from . import imag as _  # type: ignore[no-redef]