Write `developer.md` to explain maintainers process

[matthewfeickert]

Following up on #157 (comment) write up notes that explain how to take care of releases and general maintenance.

[matthewfeickert]

@eduardo-rodrigues while I still need to make a maintainers doc I recorded a video of me releasing pylhe v0.5.1 (and trying to explain what I'm doing):

(^thumbnail is a link to the YouTube video)