Clarify concepts of maintainer/committer/contributor

[cornelius]

We haven't clearly defined all of the concepts of maintainer, committer, and contributor and how they are related to each other, yet. This can be seen as a contributor funnel, where people joining the project get more responsibility with more involvement. It can also seen from the perspective of permissions different roles have in making and accepting changes. Details may vary from project to project, but it would be good to have clarity about the terms and use them consistently.

[cornelius]

• How contributions are accepted in a project depends on the project. One model could be that only maintainers are accepting pull requests, another model could be that there is a larger group of committers who do that.
• Documentation for possible contributor/committer models would be helpful.