Update documentation for new rv output

Author: simonrw

README.rst

@@ -28,14 +28,15 @@ To compute the TTV properties for one or more planets:
   - the time step for the integration in days,
   - and the end point for integration in days.
 
-The function ``ttvfast.ttvfast`` returns five lists:
+The function ``ttvfast.ttvfast`` returns a dictionary containing ``positions`` and ``rv``. The ``positions`` entry is a tuple of:
 
 1. a list of integer indices for which values correspond to which planet,
 2. a list of integers defining the epoch,
 3. a list of times,
 4. a list of rsky values, and
 5. a list of vsky values.
 
-For more details, see the original C module's documentation: https://github.com/kdeck/TTVFast/blob/master/c_version/README
+The optional ``rv_times`` parameter takes a list of RV times on which the RV is to be calculated. If so the ``rv`` entry in the output 
+dictionary is populated with a list of RV values, and otherwise ``None``.
 
-Currently the RV output portion of ``TTVFast`` is not supported.
+For more details, see the original C module's documentation: https://github.com/kdeck/TTVFast/blob/master/c_version/README