Add a Code of Conduct document

[elisechant]

The Code of Conduct document is linked from the Readme template, and could be used by all of the repos.

Need to add content to Code of Conduct guide here: https://github.com/CodeforAustralia/standards/blob/master/templates/CODE_OF_CONDUCT.md

[elisechant]

Shopify have a good Code of Conduct adapted from Contributor Covenant which we could inspire from?