Add clear contribution documentation / guidelines

[soceanainn]

Develop an explicit contribution model for this project and document it.

Code contributions can be in the form of:

• small improvements to individual games / static pages
• improvements to the website design
• adding entire new games to the website

Content contributions can be in the form of:

• adding content to individual games (e.g. adding additional 'cards' in Cards against Humanity clone)
• adding new 'Choose Your Own Adventure' stories (using this template)
• adding internationalisation rules for new minority languages

Contribution guidelines should account for the fact that non-technical users may wish to contribute content and that a Github-based workflow may not be suitable for this purpose.