Add contributing guidelines

CONTRIBUTING.md

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+## Contributing
+
+We welcome contributions from anyone, even if you are new to open source. It
+might sound daunting to contribute to a compiler at first, but please do, it is
+not complicated. We will help you with any technical issues and help improve
+your contribution so that it can be merged.
+
+To contribute, submit a Merge Request (MR) against our repository at:
+
+https://gitlab.com/lfortran/lfortran
+
+If you have any questions or need help, please ask as at our
+[mailinglist](https://groups.io/g/lfortran) or a
+[chat](https://lfortran.zulipchat.com/).
+
+Please note that all participants of this project are expected to follow our
+Code of Conduct. By participating in this project you agree to abide by its
+terms. See [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md).
+
+By submitting an MR you agree to license your contribution under
+the LFortran's BSD [license](LICENSE) unless explicitly noted otherwise.