Design notes for LBNext

[bajtos]

We need a brief architectural guide explaining high-level ideas to extension developers, few diagrams to visualize the concepts (Raymond).
https://github.com/strongloop/loopback-next/wiki/Crafting-LoopBack-Next

See #218

[Setogit]

An extension starter project ( a.k.a. loopback-next-hello-extension ) : #525