Add contributor guidelines

Author: jonsharratt

.github/CONTRIBUTING.md

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+# Contributing Guidelines
+
+Welcome, and thanks in advance for your help!
+
+## When you want to propose a new feature or bug fix
+* Please make sure there is an open issue discussing your
+contribution.
+* If there isn't, please open an issue so we can talk about it before you invest
+time into the implementation.
+* When creating an issue follow the guide that Github shows so we have enough
+information about your proposal.
+
+## Pull Requests
+Please follow these Pull Request guidelines when creating pull requests:
+* If an issue exists, leave a comment there that you are working on a solution
+so nobody else jumps on it.
+* If an issue does not exist, create a new Issue, detail your changes.  We
+recommend waiting until we accept it, so you don't waste your precious time.
+* Follow our **Testing** and **Code Style** guidelines below.
+* Start commit messages with a uppercase verb such as "Add", "Fix", "Refactor",
+"Remove".
+
+## Issues
+Please follow these issue guidelines for opening issues:
+* Make sure your issue is not a duplicate.
+* Make sure your issue is for a *feature*, *bug*, or *discussion*, use the
+labels provided in Github where applicable.
+
+## Code Style
+We aim for clean, consistent code style.  We're using ESlint to check for
+codestyle issues. If ESlint issues are found our build will fail and we can't
+merge the PR.
+
+Please follow these Code Style guidelines when writing your unit tests:
+* In the root of our repo, use this command to check for styling issues: `npm
+run lint`
+
+## Testing
+We strive for 100% test coverage, so make sure your tests cover as much of your
+code as possible.