Needs docs on module initialization

[timholy]

The README doesn't describe how to handle ParallelStencil's initialization requirements. It should mention that each dependent package needs a __init__() function that calls @init_parallel_stencil, otherwise folks may try to call it from top-level in their module. xref https://github.com/timholy/Revise.jl/issues/821

Alternatively (and better), you could switch to storing all the needed settings in the dependent module, e.g., have @init_parallel_stencil create a global const __parallel_stencil_initialized__ = true in the downstream package. It would also have to store any needed settings as well, of course. That would be precompile-friendly. The reason this is better is that (1) users don't need to know anything about __init__ functions, and (2) __init__ functions are awful black-boxes when it comes to the upcoming ability to compile small binaries (we will have to cache them regardless of whether they are needed). So it should be a win-win to move your storage into the dependent package.