Update README.md

Author: Gagniuc

README.md

@@ -12,3 +12,7 @@ Objective Digital Stains in VB6
 ![screenshot](https://github.com/Gagniuc/PromKappa-3.0-Objective-Digital-Stains-in-VB6/blob/main/screenshot/PromKappa%202.0%20Objective%20Digital%20Stain%20(5).gif)
 
 ![screenshot](https://github.com/Gagniuc/PromKappa-3.0-Objective-Digital-Stains-in-VB6/blob/main/screenshot/PromKappa%202.0%20Objective%20Digital%20Stain%20(6).PNG)
+
+# References
+
+<i>Paul A. Gagniuc. Algorithms in Bioinformatics: Theory and Implementation. John Wiley & Sons, Hoboken, NJ, USA, 2021, ISBN: 9781119697961.</i>