Added AVL Tree Data Structure and Made appropriate changes in Readme.md

[singhalpranav22]

Added AVL tree Data structure

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Contribution for hacktober fest

[bhavinjawade]

Hi @singhalpranav22,

Thanks for your contribution. 2 reasons I can't accept the pull request right away:

1. Plagiarism: Please do not use code directly from geeksforgeeks. Instead, try writing the code from scratch.
2. Please write an explanation for AVL tree and your code. Add the markdown for the same to the repository.

Let me know if you have any questions.

[singhalpranav22]

Sure..I will look forward and won't make this mistake again in future.