Contribution guidelines

Install/update the dependencies:
```
npm install
```
Make sure the code lints:
```
npm run lint
```
Make sure all of the tests pass:
```
npm test
```
Adhere to the coding conventions that are used elsewhere in the codebase.
New code must have new unit tests.
New features or changes to existing features must be documented in the readme file.
Add yourself to the authors file.
Feel free to open an issue first, if the change is one that you think needs some discussion.