Document interactions between development machine, GitHub and Redmine [ci skip]

Sequence diagram made with http://bramp.github.io/js-sequence-diagrams/, given
the following description:

    Development\nmachine -> GitHub : git push
    GitHub -> Redmine : POST /github_hook
    Redmine -> GitHub : git fetch
    GitHub --> Redmine : Commits
    Redmine -> Redmine : Update\nRepository