Add contributing guide

[c4milo]

Some examples:

• https://github.com/h5bp/html5-boilerplate/blob/master/CONTRIBUTING.md
• https://github.com/hashicorp/terraform/blob/master/CONTRIBUTING.md
• https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

[c4milo]

Landed in 59f5ac6