Update contributing docs to account for new @embroider/macros based build setup

[simonihmig]

Fixes #1124

[jelhan]

Thanks a lot for working on this one!

I think the introduction should be changed as well. It says that the project structure is different to support multiple Bootstrap versions:

This addon supports Bootstrap 3 and 4 at the same time. Therefore the project structure is a bit different than that of usual Ember addon. Please read the section on Dual Bootstrap support for more detailed explanations.

I think "project structure" does not fit well for our new approach. For me it sounds like file system layout.

Not sure if want to speak directly about "build-time macros" or Embroider. Maybe "feature flags" is a more common term? I think what we are doing here is supporting Bootstrap 3 and 4 through build-time feature flags.

From that perspective using Embroider macros or a special file system layout is an implementation detail only.

[simonihmig]

I think the introduction should be changed as well. It says that the project structure is different to support multiple Bootstrap versions:

I changed it to "build setup" if you don't mind. Feature flags seems a bit misleading for me, as I think of it as a flag to enable some kind of experimental feature, which is disabled by default. At least that's how they are used for Ember canary.