Create README.md for Counting Sort

[K-u-n-a-l-c]

This PR creates/modifies the README for

I have read the contributing guidelines in CONTRIBUTING.md and have ensured the following:

• [x] Algo name (as heading)
• [x] A small description about algo
• [x] Time Complexity (Worst, Average, Best case)
• [x] Space Complexity (Worst, Average, Best case)
• [x] Logic and Pseudocode
• [x] Instructions for running code

[K-u-n-a-l-c]

Sure! @codelixir