Document that md.pair.dipole has no shift modes
Document that md.pair.dipole has no shift modes

fixes #178

The reporter of #178 has not commented on or taken any action for
several months. I am resolving this issue by documenting the current
behavior.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue