Add contribution guidelines to CONTRIBUTING.md

Added guidelines on restreaming and user contributions.

Author: xaoscience

CONTRIBUTING.md

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+Tiny gates one can step over don't seem logical.
+So, restreaming should not be abstracted and made free for everyone, with the warning that streaming endpoints might detect it and do not always support restreaming (especially for those in a partner program).
+
+If there is anything you can do to expand, simplify or otherwise optimise the process for the user, please feel free to contribute!