Create a FAQ

[kyonifer]

There's some things that could use further explanation. In particular:

1. The codegen process, why we have it, and how to modify it for contributors (this might actually be best in a CONTRIBUTING.md?)

2. Discussion of our approach to limiting boxing via inline extension functions (see Make NDArray's map function generic #47 for discussion). Should probably include a brief history of other methods that were tried (e.g. abstract base with internal overrides on concrete implementations that specialized the primitive type)