Create CONTRIBUTING.md

To meet the community standards .

For more: https://github.com/xtekky/gpt4free/community

CONTRIBUTING.md

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+<img alt="gpt4free logo" src="https://user-images.githubusercontent.com/98614666/233799515-1a7cb6a3-b17f-42c4-956d-8d2a0664466f.png">
+
+### Please, follow these steps to contribute:
+1. Reverse a website from this list: [sites-to-reverse](https://github.com/xtekky/gpt4free/issues/40)
+2. Add it to [./testing](https://github.com/xtekky/gpt4free/tree/main/testing)
+3. Refractor it and add it to [./gpt4free](https://github.com/xtekky/gpt4free/tree/main/gpt4free)
+
+### We will be grateful to see you as a contributor!