Add the first draft of development doc.
Add the first draft of development doc.

aleksandrs-ledovskis/omnibus-gitlab,felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,gitlabhq/omnibus-gitlab,felixbuenemann/omnibus-gitlab,felixbuenemann/omnibus-gitlab,copystudy/omnibus-gitlab