Add a contributing guide

This is a draft of a contributing guide for the library.

The file is called 'CONTRIBUTING.md' and is placed in the root of the repository so that GitHub is able to pick it up and [automatically display](https://github.com/blog/1184-contributing-guidelines) to a person creating an issue or a pull request.

This draft is based on the contributing guidelines of [html5-boilerplate](https://github.com/h5bp/html5-boilerplate/blob/master/CONTRIBUTING.md) project. It also includes a few borrowings from [homebrew-cask](https://github.com/phinze/homebrew-cask/blob/master/CONTRIBUTING.md).

The guide involves a short checklist that the maintainers may refer contributors to.

Perhaps a few notes about tests may be added, though I am not sure what the actual guidelines in that respect are. Anyone knowledgeable enough is very welcome to augment the draft. Comments are welcome as well.