Add diagrams from draw.io (#122)

* Add diagrams from draw.io

I'm trying to figure out how best to share these in such a way that we
can all contribute to them easily. While integration between draw.io
and GitHub exists, it requires read/write permissions to all ParityTech
repos, which doesn't comfort me. I think having the .drawio file and
the corresponding pictures available in a folder is a good start.

I imagine the workflow would be something like this: Someone wants to
update a diagram, so they go to draw.io and upload the .drawio file
from the repo. They make their changes, download the new .drawio file
and corresponding SVG, and commit their changes to the repo.

* Make diagrams consistent with background/filling

The background for each diagram has been removed (now it's transparent)
and the boxes that make up the diagrams have gotten a filled in white
colour. This will make it possible to have these diagrams show up on
non-white backgrounds.

* Add file for describing diagrams

* Add missing extension