docs: add explanation of the ranks (anuraghazra#576)

readme.md

@@ -73,7 +73,9 @@ Change the `?username=` value to your GitHub's username.
 [![Anurag's github stats](https://github-readme-stats.vercel.app/api?username=anuraghazra)](https://github.com/anuraghazra/github-readme-stats)
 ```
 
-_Note: Ranks are calculated based on user's stats, see [src/calculateRank.js](./src/calculateRank.js)_
+_Note: Available ranks are S+ (top 1%), S (top 25%), A++ (top 45%), A+ (top 60%), and B+ (everyone).
+The values are caluclated by using the [cumulative distribution function](https://en.wikipedia.org/wiki/Cumulative_distribution_function) using commits, contributions, issues, stars, pull requests, followers, and owned repositories.
+The implementation is can be investigated at [src/calculateRank.js](./src/calculateRank.js)_
 
 ### Hiding individual stats