contributor code of conduct

Change-Id: Id543e945fdfe721493bfd1470d12b26b1916cca3
Reviewed-on: https://gerrit.instructure.com/46252
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>