Add the role of a Package Developer

[leouieda]

Create a role between maintainer and contributor which reflects how we operate in practice. Developers have write access to the repository and certain responsibilities which we currently assign informally. This formalizes the role. Also separate the "roles" from "titles" (author, contributor, founder) which have no responsibilities attached are there for recognition only.

[leouieda]

@fatiando/council what do you think?

[santisoler]

I like it! I think the new Package Developer role will facilitate promoting active contributors.

I just pushed some minor edits to the text (mostly typos/grammar errors).

[leouieda]

Thanks, @santisoler and @mdtanker ❤️