Update README.md

Author: financialnoob

README.md

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 # portfolio_optimization
 experiments with portfolio optimization
 
-1. https://financialnoob.me/introduction-to-portfolio-optimization-and-modern-portfolio-theory/
-2. https://financialnoob.me/portfolio-optimization-with-weighted-mean-and-covariance-estimators/
-3. https://financialnoob.me/portfolio-optimization-random-matrix-approach-to-estimating-covariance-matrix/
+substack:
+1.