Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Contributing
+
+Contributions to Ward are encouraged and very welcome!
+
+Contributions can come in many forms: documentation enhancements, features, bug fixes, creating issues, and participating in the community.
+
+If you're interested in helping out, you might find some inspiration in [Issues](https://github.com/darrenburns/ward/issues). If you have an idea, but don't see it there, don't hesitate to create a new issue.
+
+Before submitting a pull request, please make sure the enhancement or bugfix you've made has been discussed. 
+
+This will ensure no work is duplicated, and that a general approach has been agreed.
+
+Please also take time to review the [Code of Conduct](https://github.com/darrenburns/ward/blob/master/.github/CODE_OF_CONDUCT.md). Anyone who violates the code of conduct may be barred from contributing.
+
+## Pull request guidelines
+
+When creating a PR:
+
+* Code is formatted using `black -l 120 .` (`Makefile` coming soon)
+* Ensure `README.md` is updated if necessary.
+* Tests written cover new code, and running `ward` results in a pass.
+
+If something is missing from this guide (it probably is), please let me know by creating an issue or a pull request.