docs: how to opt out of precompiled artefacts

[cocoa-xu]

Hi @warmwaffles, I split the Configuration section into two subsections because I'd like to make it clearer that make_force_build is a compile-time option (so that users won't get confused about where they should set it).

But please let me know if you have a better idea of which section I should mention this. :D

[warmwaffles]

That's a good change. I wish there was a way to detect that a variable was modified at runtime that is compile time only and Kernel.warn developers.

[cocoa-xu]

That's a good change. I wish there was a way to detect that a variable was modified at runtime that is compile time only and Kernel.warn developers.

Yeah, I wish elixir has this feature as well.