Update README.md with A/B testing calculators

Author: erictleung

README.md

@@ -77,6 +77,7 @@ Curated resource materials from around the internet for data science, with appli
 - [Modern Statistics for Modern Biologists][msmb]
 - [An Introduction to Statistical Learning](https://www.statlearning.com/) - With editions in R and Python.
 - [Library of Statistical Techniques (LOST)](https://lost-stats.github.io/) - Publicly-editable website with the goal of making it easy to execute statistical techniques in statistical software.
+- [List of A/B Testing Calculators](https://www.evanmiller.org/ab-testing/)
 
 [peerjstats]: https://peerj.com/collections/50-practicaldatascistats/
 [topcrossvotes]: https://stats.stackexchange.com/questions?sort=votes