add rigid body
+Rigid Body
+==========
+
+.. automodule:: elastica.rigidbody.rigid_body
+ :members:
+ :exclude-members: __weakref__
+
+.. automodule:: elastica.rigidbody.cylinder
+ :members:
+ :exclude-members: __weakref__
+
+.. automodule:: elastica.rigidbody.sphere
+ :members:
+ :exclude-members: __weakref__
