Subclass discovery depends on compilation order

I think this is yet another macro bitten by the infamous [SI-8776](https://issues.scala-lang.org/browse/SI-8776 - discussion [here]%28https://groups.google.com/forum/#!topic/scala-user/oJP9aqs88rM%29). A potential pragmatic workaround is to shuffle classes around until it forces the compilation order to be correct, i.e. to have potential subclasses been typed before the macro is invoked.

I suppose that until the issue is closed it would perhaps be a good idea to include this in the docs, under pitfalls or so.

Start by reading the compilation-order discussion linked in the issue and inspect the repository's existing documentation structure. Document the subclass-discovery compilation-order pitfall and the suggested class-order workaround under an appropriate pitfalls section, with the SI-8776 context included.

Written by the indexing model from the issue text.