Add CONTRIBUTING.md (#406)

CONTRIBUTING.md

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+# Contributing to Material-UI X
+
+## Material-UI X vs. Material-UI
+
+Material-UI X is an extension of Material-UI.
+The repositories are part of the same codebase.
+`mui-org/material-ui-x` imports the code infrastructure from [`mui-org/material-ui`](https://github.com/mui-org/material-ui).
+You can find the "contributing" guide for the main repository in [mui-org/material-ui/CONTRIBUTING.md](https://github.com/mui-org/material-ui/blob/HEAD/CONTRIBUTING.md).
+
+Most of the content in the Material-UI "contributing" guide applies to this repository.
+There are, however, a few differences:
+
+- The default GitHub branch might be different.
+- The local documentation site opens on a different port: 3001 instead of 3000.