Added the simple documentation of the cstr using readme.

gdplib/cstr/README.md

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+# GDP Reactor Series Design
+Function that builds CSTR superstructure model of size NT (default = 5). 
+NT is the number of reactors in series.
+The CSTRs have a single 1st order auto catalytic reaction A -> B and minimizes total reactors series volume. 
+The optimal solution should yield NT reactors with a recycle before reactor NT.
+
+Reference:
+> Linan, D. A., Bernal, D. E., Gomez, J. M., & Ricardez-Sandoval, L. A. (2021). Optimal synthesis and design of catalytic distillation columns: A rate-based modeling approach. Chemical Engineering Science, 231, 116294. https://doi.org/10.1016/j.ces.2020.116294
+
+### Solution
+
+Best known objective value: 3.06181298849707
+
+### Size
+
+Number of reactors in series is 5.
+
+| Problem   | vars | Bool | bin | int | cont | cons | nl | disj | disjtn |
+|-----------|------|------|-----|-----|------|------|----|------|--------|
+| gdp_reactors | 71 | 15 | 0 | 0 | 56 | 25 | 2 | 20 | 10 |
+
+- ``vars``: variables
+- ``Bool``: Boolean variables
+- ``bin``: binary variables
+- ``int``: integer variables
+- ``cont``: continuous variables
+- ``cons``: constraints
+- ``nl``: nonlinear constraints
+- ``disj``: disjuncts
+- ``disjtn``: disjunctions