add citation of geppy

README.MD

@@ -70,4 +70,19 @@ The bible of GEP is definitely Ferreira, C.'s monograph: **Ferreira, C. (2006).
 You can also get a lot of papers/documents by Googling 'gene expression programming'.
 
 [1] Ferreira, C. (2001). Gene Expression Programming: a New Adaptive Algorithm for Solving Problems. Complex Systems, 13.
-[2] Zhong, J., Feng, L., & Ong, Y. S. (2017). Gene expression programming: a survey. IEEE Computational Intelligence Magazine, 12(3), 54-72.
+[2] Zhong, J., Feng, L., & Ong, Y. S. (2017). Gene expression programming: a survey. IEEE Computational Intelligence Magazine, 12(3), 54-72.
+
+## How to cite
+If you find *geppy* useful in your projects, please cite it such that more researchers/engineers will know it. A BibTeX entry for *geppy* is given below.
+```
+@misc{geppy_2020,
+    author       = {Shuhua Gao},
+    title        = {{geppy: a framework for gene expression programming }},
+    month        = July,
+    year         = 2020,
+    doi          = {10.5281/zenodo.3946275},
+    version      = {0.1.1},
+    publisher    = {Zenodo},
+    url          = {https://github.com/ShuhuaGao/geppy}
+    }
+```