Add third option to wind_turbine_fleet parameter and restructure the section

Author: uvchik

windpowerlib/wind_farm.py

@@ -22,17 +22,22 @@ class WindFarm(object):
     Parameters
     ----------
     wind_turbine_fleet : :pandas:`pandas.DataFrame<frame>` or list()
-        Wind turbines of wind farm. DataFrame must have
-        'wind_turbine' containing a :class:`~.wind_turbine.WindTurbine` object
-        and either 'number_of_turbines' (number of wind turbines of the same
-        turbine type in the wind farm, can be a float) or 'total_capacity'
+        Wind turbines of the wind farm.
+
+        1. DataFrame must have 'wind_turbine' containing a
+        :class:`~.wind_turbine.WindTurbine` object and either
+        'number_of_turbines' (number of wind turbines of the same turbine type
+        in the wind farm, can be a float) or 'total_capacity'
         (installed capacity of wind turbines of the same turbine type in the
         wind farm) as columns/keys. See example below.
 
-        A list of :class:`~windpowerlib.wind_turbine.WindTurbineGroup` objects.
-        A WindTurbineGroup can be created from a
+        2. A list of :class:`~windpowerlib.wind_turbine.WindTurbineGroup`
+        objects. A WindTurbineGroup can be created from a
         :class:`~windpowerlib.wind_turbine.WindTurbine` using the
         :func:`~windpowerlib.wind_turbine.WindTurbine.to_group` method.
+
+        3. It is still possible to use a list of dict (see example) but we
+        recommend to use one of the other options above.
     efficiency : float or :pandas:`pandas.DataFrame<frame>` or None (optional)
         Efficiency of the wind farm. Provide as either constant (float) or
         power efficiency curve (pd.DataFrame) containing 'wind_speed' and