Create CONTRIBUTING.md

Author: MichelML

CONTRIBUTING.md

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+## Contributing
+
+You are welcome to contribute to [any GitHub issue](https://github.com/rdkit/rdkit-js/issues) currently open.
+
+You are also welcome to [log any issue](https://github.com/rdkit/rdkit-js/issues/new/choose) or [start a discussion](https://github.com/rdkit/rdkit-js/discussions/new) on any topic related to RDKit.js.
+
+To contribute to the plain javascript examples, read [this sub-repository README](https://github.com/rdkit/rdkit-js/tree/master/examples/javascript).
+
+To contribute to the React.js examples, read [this sub-repository README](https://github.com/rdkit/rdkit-js/tree/master/examples/react).