C++ implementation for greedy graph colouring algo

[abhaysinghraihal]

This PR implements the code for in referred in issue #

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

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This PR creates/modifies the README for

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

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