Update README.md

Author: financialnoob

README.md

@@ -4,3 +4,4 @@ experiments with portfolio optimization
 medium:
 1. https://medium.com/@financialnoob/introduction-to-portfolio-optimization-and-modern-portfolio-theory-101f663829c4
 2. https://medium.com/@financialnoob/portfolio-optimization-with-weighted-mean-and-covariance-estimators-de6dda2cab3d
+3. https://medium.com/@financialnoob/portfolio-optimization-random-matrix-approach-to-estimating-covariance-matrix-e1cca4554466