Create CONTRIBUTING.md

Author: mdavids

CONTRIBUTING.md

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+# Contributing
+
+SPIN uses GitHub to manage reviews of pull requests.
+
+* If you have a trivial fix or improvement, go ahead and create a pull
+  request.
+  
+* If you find a bug, like to suggest an enhancement or have any other issue, please create an issue in Github.
+
+* If you plan to get otherwise involved, for example discuss your ideas, there is also a
+  [mailing list](https://mailman.sidn.nl/cgi-bin/mailman/listinfo/spin).
+  This will avoid unnecessary work and surely give you and us a good deal
+  of inspiration.