Create CONTRIBUTING.md

Author: hanshulll

CONTRIBUTING.md

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+<h2 align=center> HacktoberFest 2021 :fire:</h2>
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+<p align="center">
+    <img src="https://i.ibb.co/cgNjFRR/hacktober.png" width="100%" height="100%">
+
+</p>
+
+
+>- HacktoberFest is back again. Showcase your coding skills by creating PRs and contributing to the Open Source.
+
+#### [hacktoberfest 2021](https://hacktoberfest.digitalocean.com/)
+
+
+### Let's Contribute :+1:
+- **Step 1** - Fork this repository.
+- **Step 2** - Clone the repository to your local machine.
+- **Step 3** - Resolve the bugs, mentions provided in the Issues section of the repository. *Also add a description of what changes you have done*.
+- **Step 4** - Add the changes to your repository.
+- **Step 5** - Create a PULL Request. And that's all.
+- **NOTE** - Please start the filename with the platform name on which the problem was solved. For eg if you are submitting graph problem from codechef then mention filename as **(CODECHEF) chefandgraph.cpp**.
+
+### What you can contribute in this repo? :punch:
+- You can add your own competitive programming solutions or coding related solution. 
+- There are various topics like Recursion , Bits Manipulation, Graphs etc. ***You can contribute in them***
+- You can contribute some **Learning Resources** in the ***Readme.md*** File.
+- You can modify previous solutions if you feel like your solution has better ***Time or Space complexity.***
+
+
+##### Let's Contribute :smiley: