suggestion: move the Implementation section of the README into generator_script.jl

[digital-carver]

Having it in the Readme makes us think it's relevant to the package user (even if the section is titled "Implementation"), and reading it made me uncertain about whether this is a dev-only dependency used to generate code, or if it uses @eval on the strings it's talking about, etc. Looking at the code, it becomes obvious that it's a casual remark about an implementation detail, but it would be better to avoid the potential for confusion in the first place.