Add contributor guide

[fifieldt]

Many git repos have a dedicated CONTRIBUTING.md file that explain how to contribute in a bit more depth. Creating one of those is a good way to capture some of our 'tribal knowledge' about how we work together, making it easier for new people to jump in and feel confident they are doing the 'right' thing. To cover areas such as:

• Who can get push rights, and who to contact to get them (at the moment, all comers should say hi and are given push rights)
• How we communicate outside of github (ie our LINE group)
• Encourage new contributors to add their name to about.md
• When it's OK to merge a patch straight into master (deployment, typo fixes, non-contentious content updates, straightforward layout updates) and when we like to use an issue or pull request to review (updating/adding FAQ entries which could be contentious or it's difficult to say for sure we're correct, big layout changes)
• Our collaboration 'principles'
• how to find something to work on

[geekyme]

On top of this one, probably its also a good idea to put in tools for code formatting / linting, etc. This way there can be some consistency between all contributors.