Update README.md

Adding doi for the code.

README.md

@@ -27,3 +27,9 @@ The paper is available at www.mdpi.com/1424-8220/16/6/790/pdf and a simple bibte
   publisher={Multidisciplinary Digital Publishing Institute}
 }
 ```
+If using the code in your research please directly **cite the code**:
+Merima Kulin, Carolina Fortuna, Eli De Poorter, Dirk Deschrijver and Ingrid Moerman (2016, December 28), Scripts used for solving a wireless device fingerprinting classification problem in the "Data-Driven Design of Intelligent Wireless Networks: An Overview and Tutorial". GitHub. Retrieved from https://github.com/merimak/DataDrivenDesignWirelessNetworks, doi: 10.5281/zenodo.223332.
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+[![DOI](https://zenodo.org/badge/54724227.svg)](https://zenodo.org/badge/latestdoi/54724227)
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