State Pandas dependency optional

Author: lindonroberts

README.rst

@@ -59,7 +59,7 @@ Additionally, the following python packages should be installed (these will be i
 
 * NumPy 1.11 or higher (http://www.numpy.org/)
 * SciPy 0.18 or higher (http://www.scipy.org/)
-* Pandas 0.17 or higher (http://pandas.pydata.org/)
+* Optionally, Pandas 0.17 or higher (http://pandas.pydata.org/), to return the diagnostic information as a DataFrame
 
 **Optional package:** Py-BOBYQA versions 1.2 and higher also support the `trustregion <https://github.com/lindonroberts/trust-region>`_ package for fast trust-region subproblem solutions. To install this, make sure you have a Fortran compiler (e.g. `gfortran <https://gcc.gnu.org/wiki/GFortran>`_) and NumPy installed, then run :code:`pip install trustregion`. You do not have to have trustregion installed for Py-BOBYQA to work, and it is not installed by default.