Update README.md

tutorials/optimization/README.md

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 ## Optimization code
 
-- chr_NL_optimization: optimization for the chromosome-nuclear landmarks interactions
+When developing the nucleus model, we optimized the interaction parameters between chromosomes and those between chromosome-nuclear landmarks separately. These optimizations make use of experimental data for HFF cells, and the resulting model is cell-type specific. When applying the model to a different cell type, one may consider reoptimizing the parameters with corresponding experimental data. The two folders here provide details on how to carry out parameter optimization.  
 
-- chr_chr_optimization: optimization for the chromosome-chromosome interactions
+- chr_NL_optimization: parameter optimization for chromosome-nuclear landmarks interactions
+
+- chr_chr_optimization: parameter optimization for chromosome-chromosome interactions