Create an architecture document

[jodh-intel]

We need to create an ARCHITECTURE.md that covers the overall system.

This document could, if necessary, reference architecture documents found in each components repository. These would essentially document just themselves. The intent is to define once and reference as many times as needed.

For example, the ARCHITECTURE.md created in this repository may say something like:

For further details on the Kata shim implementation, refer to the shim architecture document

Component repositories

The individual README.md files in each code repository must contain a reference to this central architecture document.

If a component repository has a separate ARCHITECTURE.md this:

• Must be referenced in the components README.md.
• Must itself reference the central ARCHITECTURE.md document.

For an example of what we should be aiming for, see the Clear Containers architecture document:

• https://github.com/clearcontainers/runtime/blob/master/docs/architecture/architecture.md

Diagrams

All diagrams must:

• Be stored along with their corresponding "source" files (for example, if a .png file is referenced, the .svg file that was used to generate the .png must also be stored to allow the diagrams to be updated by all.
• Use freely available open formats for their sources.

[grahamwhaley]

@jodh-intel - is this Issue satisfied by https://github.com/kata-containers/documentation/blob/master/architecture.md ?

[jodh-intel]

Technically, no, as we're missing the sources for the PNG files.

@egernst - could you rustle up the sources for the PNG files here: https://github.com/kata-containers/documentation/tree/master/arch-images ?