Added CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute to Mantis?
+
+If you want to contribute to Mantis:
+
+- Submit an issue if you found a bug, or a have a feature request
+- Make a Pull Request from dev branch if you want to improve the code
+- Use [Mantis Discussions - Contributions](https://github.com/PhonePe/mantis/discussions/categories/contribution) forum to seek help regarding contributions