Add architectural-direction.md

[Laurentjg]

The Readme.md describes that some architectural decisions are written in the architecture section.
Add the decisions so that visitors and contributors understand how we are going to integrate and what the deliverable will be in the next stage. (e.g. decisions on how both software stacks are integrated).

[Laurentjg]

Still relevant to describe the architecture for building a general understanding on the work delivered