update whatsnew

Author: cwhanse

docs/sphinx/source/whatsnew/v0.6.1.rst

@@ -24,6 +24,9 @@ API Changes
 * Changed function name from :py:func:`pvlib.solarposition.get_rise_set_transit` (deprecated)
   to :py:func:`pvlib.solarposition.sun_rise_set_transit_spa. `sun_rise_set_transit_spa`
   requires time input to be localized to the specified latitude/longitude. (:issue:`316')
+* Changed function :py:func:`pvlib.pvsystem.sapm` to expect input `effective_irradiance`
+  in units of W/m2 rather than suns. Changed output of function 
+  :py:func:`sapm_effective_irradiance` to units of W/m2 rather than suns.
 
 
 Enhancements