Building a developer community

[jedbrown]

This document currently focuses on technical aspects of development, but I think it would be valuable to augment with best practices for building a community. Recruiting users is important, but in this issue I would like to focus on developers. We should make recommendations for bringing users into the development community and providing a welcoming environment for contributions. This can include software architecture (making extensions easy to develop and contribute), documenting accessible projects, mentoring, and overall culture.