Add a CONTRIBUTING.md file

[kieran-ryan]

A CONTRIBUTING.md file is an important element of every repository. It provides visitors with an overview of how they can set up, work with and contribute to the repository. Writing this documentation, also provides repository owners with a useful opportunity to assess usability issues and to examine the project from a fresh perspective, with a mindset of 'What issues might someone seeing this for the first time encounter?' or 'Is there any way I could make this implementation more explicit and reduce its "unknown unknowns"?'.

As this is a template that will form the basis of other repositories, it is important that it provides a well-documented contributing guide that captures questions and issues that a new visitor may encounter. This will allow them to act with autonomy and to not be dependant on direct support and answers from the original authors.