[Meta] Contributor agreement, code of conduct, acknowledgements

We need to sort out how contributors are acknowledged, any transfer of IP rights. At the same time, let's sort out code of conduct and other contributor bits and pieces. We have a limited set of information at https://keyman.com/about/get-involved.

react's 'how to contribute' may be a good place to start: https://reactjs.org/docs/how-to-contribute.html#contributor-license-agreement-cla