Add problem description for 10 Days of Statistics

Currently, there is no problem description file for [10 Days of Statistics](https://github.com/Kvaibhav01/HackerRank-Solutions/tree/master/10%20Days%20of%20Statistics).

Add a _README.md_ file for each day problem explaining how to solve the issue, and if possible what the solution code means.

Example: For [Day 0: Mean, Median and Mode](https://github.com/Kvaibhav01/HackerRank-Solutions/tree/master/10%20Days%20of%20Statistics/Day%200-%20Mean%2C%20Median%2C%20and%20Mode), add the README file that explains above so that we have two files in the Day 0 folder; one of the actual solution and second the README file.

Start in the 10 Days of Statistics directory and inspect each day's existing Java solution, beginning with Day 0: Mean, Median and Mode. Add a README.md to every day folder explaining the problem, approach, and solution code; done means all ten day folders contain both the solution and README files.

Written by the indexing model from the issue text.