Gather code design decisions in CodeDesign.md

[mknorps]

As mentioned by @Nour-Mws in https://github.com/tweag/FawltyDeps/pull/47#discussion_r1065645516, Code Design document right now focus on Code Style.

The task is to add information on code design decisions made:

• a clear description of requirements (incl supported Python version, minimal external dependencies)
• external constraints (PEP's, capabilities of packages in the ecosystem and std library)
• code architecture (structure of folders and basic building blocks of the project, separations of concerns, usage of functional/OOP style, generators, ...)
• user experience
• developer experience

other? @Nour-Mws @jherland