README: Add a Tree Diagram of the Assembly Decomposition

[StephenCarlson]

The assembly is super confusing right now. All these _A and _B parts, why is there only a Left side of anything? Which parts need to be printed? These are all things that are either barely documented in the #airframe-parts- section, or are implicit in the name of the STLs. Here is a quick synopsis:

• For parts such as the Wings and Vertical Stabilizers and Winglets, only the left STL is included, since these are easily mirrored to the Right-side version in whatever slicer these are pulled into. There is no point to including a Right-side of each of these STLs since it increases the number of parts in the repository, and increases the number of steps for each release. But.... you know what, at this point, whatever, data is cheap, right? Fine, I'll add a Right of every STL, [grumbling intensifies as I add this to the ToDo list].
• For pieces that are non-chiral non-symmetric (cannot be mirrored self-similarly), these have a _A and _B suffix in the name. The Nacelles are definitely non-chiral non-symmetric, as one side (the outer or closest to wingtip) is the side that has the geometry for the servo linkage, and the inside (nearest the fuselage) is "normal". This convention is also used where pieces that are available as one piece are split into two pieces for convenience for those with smaller 3D printers. The Nose and Wings have this, where if you cannot print the whole thing, there are the _A and _B options.
• A graphical decomposition needs to be made using my CAD system's assembly artwork annotation functions. I need to edit the assembly where all the parts are animated/exploded to show relationships. More work....
• The Tree Diagram should use ASCII characters and look nice in both the raw Markdown text, and as rendered on GitHub.

Here is an example, or root node, of the Tree Diagram:

MiniHawk-VTOL Version 2.1 Aircraft
| -- Fuselage/Body
| |-- Empennage
| |-- Nose
|-- Left Wing
| |-- Left Fin
| | |-- MH7_FinLeft_Lower.stl
| | |-- MH7_FinLeft_Upper.stl
|-- Right Wing
| |-- Right Fin
| | |-- MH7_FinLeft_Lower.stl (Mirrored)
| | |-- MH7_FinLeft_Upper.stl (Mirrored)

[LilBub]

Hi Steve, I think you'll make the build easier for others who aren't engineering / student types if you include everything needed once. No mirrored parts since, for some not savvy with 3D slicers, could get it wrong. So, as I do with my technical documentation (research 3D printing for military projects), make the outline as clear and concise as possible. Also, know you audience may be less smart as you =)