Define a pygwinc contributions policy

CONTRIBUTIONS.md

+# Contributions to pygwinc
+
+The pygwinc project welcomes your contributions.  Our policy is that all contributions should be peer-reviewed.  To facilitate the review, please do not commit your work to this repository yourself.  Instead, fork the repository and send a merge request.
+
+If your change affects the shape of a noise curve, your commit message should make note of that, and provide a justification.  It will also be necessary to update the reference curves stored in DCC entry [T18xxxxx](https://dcc.ligo.org/LIGO-T18xxxxx), after the change is applied.
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