Create README.md

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

This PR creates/modifies the README for Counting sort as referred in #69

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]

I have updated the file, Please Re-review it. @deutranium Thank you.