Write Contributor Guidelines for repositories

[barrosgeraldo]

Hello Rust Bridge team,

I'm from the 2018 Brazil Rust Roadshow team, since we have engaged mobilizers to teach Rust locally throughout the country we would like to add our resources, expertise and learnings, but within this organization/repositories we do not know where to start.

We recommend write the Contributor Guidelines, you can get guidance here to do it.

Thank you so much,
Geraldo Barros
on behalf of 2018 Brazil Rust Roadshow team 🎉