Document caveats

[adrhill]

The repository description mentions that this project is "experimental".
What are the caveats/limitations that should be considered before using CasetteOverlay?

[gdalle]

@aviatesk @Keno do you have any wisdom to share about this? It might come in very handy for sparse autodiff

[MilesCranmer]

Ping on this @aviatesk @Keno. This would be useful for overlaying different AD backends in DifferentiationInterface, such as when only one backend can handle GPU kernels, say.

x-ref chalk-lab/Mooncake.jl#648