updated docstring

pypfopt/base_optimizer.py

@@ -1,4 +1,10 @@
-# TODO module docstring
+"""
+The ``base_optimizer`` module houses the parent classes ``BaseOptimizer`` and 
+``BaseScipyOptimizer``, from which all optimisers will inherit. The later is for
+optimisers that use the scipy solver.
+Additionally, we define a general utility function ``portfolio_performance`` to
+evaluate return and risk for a given set of portfolio weights.
+"""
 
 import numpy as np
 import pandas as pd