Added the small batch on the gdplib

gdplib/small_batch/README.md

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+## gdp_small_batch.py
+
+The gdp_small_batch.py module contains the GDP model for the small batch problem based on the Kocis and Grossmann (1988) paper.
+
+The problem is based on the Example 4 of the paper.
+
+The objective is to minimize the investment cost of the batch units.
+
+The solution is 167427.65711.
+
+### References
+
+[1] Kocis, G. R.; Grossmann, I. E. Global Optimization of Nonconvex Mixed-Integer Nonlinear Programming (MINLP) Problems in Process Synthesis. Ind. Eng. Chem. Res. 1988, 27 (8), 1407-1421. https://doi.org/10.1021/ie00080a013
+
+[2] Ovalle, D., Liñán, D. A., Lee, A., Gómez, J. M., Ricardez-Sandoval, L., Grossmann, I. E., & Bernal Neira, D. E. (2024). Logic-Based Discrete-Steepest Descent: A Solution Method for Process Synthesis Generalized Disjunctive Programs. arXiv preprint arXiv:2405.05358. https://doi.org/10.48550/arXiv.2405.05358