Add note clearksky Ineichen term b (#814)

Thanks!

Author: cedricleroy

pvlib/clearsky.py

@@ -114,8 +114,10 @@ def ineichen(apparent_zenith, airmass_absolute, linke_turbidity,
     # reinsert tl nans
     ghi = cg1 * dni_extra * cos_zenith * tl / tl * np.fmax(ghi, 0)
 
-    # BncI = "normal beam clear sky radiation"
+    # From [1] (Following [2] leads to 0.664 + 0.16268 / fh1)
+    # See https://github.com/pvlib/pvlib-python/pull/808
     b = 0.664 + 0.163/fh1
+    # BncI = "normal beam clear sky radiation"
     bnci = b * np.exp(-0.09 * airmass_absolute * (tl - 1))
     bnci = dni_extra * np.fmax(bnci, 0)