* Basic support for C4 model primitives.

* Use the "rect" shape for nodes

With the record shape we used before, graphviz would trip over
edges that set constraint=False.

* Adopt C4 terminology: Rename Dependency -> Relationship

* Adopt C4 terminology: Rename type -> technology

* Extract a shared C4Node

This makes the code more DRY, but also allows to add company-
specific extensions more easily. One need we have is to slightly
adapt the terminology. At Spotify, we happen to call `Container`
a `Component` for example. This is now easier to implement on top
of the shared `C4Node`.

* Add "C4" shield to the README

* Document how to produce a C4 diagram