Add ProMod3 paper reference to NOTICE file (Apache license related)

NOTICE

@@ -7,5 +7,8 @@ The main authors are: Gabriel Studer, Gerardo Tauriello, Stefan Bienert,
 Marco Biasini, Niklaus Johner
                              
 If you find this software useful, please cite:
-TO BE PUBLISHED
+
+Studer, Gabriel; Tauriello, Gerardo; Bienert, Stefan; Biasini, Marco; Johner, 
+Niklaus; Schwede, Torsten (2021). ProMod3 - A versatile homology modelling 
+toolbox. PLoS computational biology, 17 (1), e1008667.