add 144. Binary Tree To Post Order Traversal and update readme

piyushsharma220699 / LeetCode-Problems-Solution-Book

REPOSITORY EXCLUDED FROM HACKTOBERFEST 2021, CHECK THIS : https://github.com/piyushsharma220699/Hacktoberfest-2021/issues/261

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Open Samarth-Gupta-2909 opened 2 years ago

Samarth-Gupta-2909 commented 2 years ago

@piyushsharma220699 kindly review it please.