Migrate content from multiple sources about "Contributing"

Created by: seancolsen

There's some good documentation here about contributing to CiviCRM using GitHub.

https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md

I'd like to move this content into the Dev Guide. @totten is it important that we keep that CONTRIBUTING.md file in place? If so, then can we wipe its contents and instead use some brief text that links readers to the new content within the Dev Guide?