Contributing guidelines

Adding a `CONTRIBUTING.md` per swcarpentry/website#69.

The [contributing guidelines](https://github.com/jiffyclub/boot-camps/blob/contributing/CONTRIBUTING.md) cover the signed-off-by, repo licenses, and preferred file formats. Anything else that should go in here?

@wking, it'd be great if you could look over the signed-off-by section.

I also updated the [README](https://github.com/jiffyclub/boot-camps/blob/contributing/README.md).