Update temperature.py

just some documentation changes

Author: MDKempe

pvdeg/temperature.py

@@ -117,8 +117,12 @@ def cell(weather_df, meta, poa=None, temp_model="sapm",
         between weather database (e.g. NSRDB) and the tempeature model (e.g. SAPM)
         The NSRDB provides calculations at 2 m (i.e module height) but SAPM uses a 10 m height.
         It is recommended that a power-law relationship between height and wind speed of 0.33 
-        be used. This results in a wind speed that is 1.7 times higher. It is acknowledged that 
+        be used*. This results in a wind speed that is 1.7 times higher. It is acknowledged that 
         this can vary significantly. 
+    
+    R. Rabbani, M. Zeeshan, "Exploring the suitability of MERRA-2 reanalysis data for wind energy 
+        estimation, analysis of wind characteristics and energy potential assessment for selected
+        sites in Pakistan", Renewable Energy 154 (2020) 1240-1251.
 
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