Add CONTRIBUTING.md (#1786)

.github/CONTRIBUTING.md

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+# Contributing
+
+TorchGeo is an open-source project built by contributors like you from around the world. If you find a bug or would like to add a new feature, please open a pull request. For further information on how to contribute, including topics like:
+
+* using git,
+* licensing requirements,
+* writing and running unit tests,
+* running linters,
+* building the documentation, and
+* adding new datasets,
+
+please see our [Contributing Guide](https://torchgeo.readthedocs.io/en/stable/user/contributing.html).
+
+You can find a curated list of issues that we believe are easy for new contributors to tackle at https://github.com/microsoft/torchgeo/contribute.