Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to the Geodynamic World Builder
+
+The Geodynamic World Builder is a community project that lives by the participation of its
+members — i.e., including you! It is our goal to build an inclusive
+and participatory community so we are happy that you are interested in
+participating!
+
+We describe the process of contributing to the Geodynamic World Builder in
+our online documentation at:
+
+https://gwb.readthedocs.io/en/latest/developer_manual/start_devel_and_contrib/index.html