Issue title: i18n guidelines
 Issue body: Hi Tom,

I want to translate this resource to pt-BR, do you have any guideline for that to be integrated with your rep?

 Comments: 
 Comment 0: Awesome, [added a note about translations](https://github.com/tmcw/mapschool/blob/gh-pages/CONTRIBUTING.md#translation) with a first approach to try, but open to any better ways to do it that still keep it simple!

 Comment 1: Great, thanks a lot for this resource. You think it would be ok to republish on our project, [Geojournalism Handbook](http://geojournalism.oeco.org.br/)?

 Comment 2: Absolutely, go for it!

 Comment 3: Cool, ping back if there's anything to add in terms of translations.

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