Add contributing policy file

CONTRIBUTING.md

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+# Contributing
+
+All contributions are assumed to be licensed under the same license as the source, i.e. CC BY-SA. This license must remain in all derivatives of this work.
+
+## Issues
+
+If you find a mistake, bug or other problem, please [open an issue](https://github.com/raspberrypilearning/robobutler/issues) in this repository.
+
+## Pull Requests
+
+If you fix a mistake, bug or problem or have something to contribute, please create a pull request for each modification. Please consider grouping modifications sensibly, i.e. don't bundle typo fixes in the same pull request as code changes, instead file them separately.
+
+Please note that sometimes things are done for pedagogical reasons so changes which make sense from a software engineering perspective (reducing duplication or making use of more advanced programming language features) may not be suitable to maintain the intended educational value.
+
+## Derivatives
+
+The licence must remain in all derivatives of this work.
+
+## Licence
+
+Unless otherwise specified, everything in this repository is covered by the following licence:
+
+[![Creative Commons Attribution 4.0 International Licence](http://i.creativecommons.org/l/by-sa/4.0/88x31.png)](http://creativecommons.org/licenses/by-sa/4.0/)
+
+***Robo Butler*** by the [Raspberry Pi Foundation](http://www.raspberrypi.org) is licensed under a [Creative Commons Attribution 4.0 International Licence](http://creativecommons.org/licenses/by-sa/4.0/).
+
+Based on a work at https://github.com/raspberrypilearning/robobutler