Create an FAQ section explaining `targets`

[jakemac53]

It is easy today to accidentally override the default target with one that doesn't contain all sources (maybe it contains only the builder sources, or the sources you want to run your custom builder on). However if you do this then downstream users won't be able to read anything in your lib dir. It isn't super obvious that you actually did this though.

We could also warn if the default target doesn't include any lib files?