README: clarify how to implement _compound_ materials

README.md

@@ -227,6 +227,16 @@ to add a new material:
 - for `WgpuBackend` simulations: do the above and add a spirv entry-point specialized to your material. scroll to the bottom of
 [crates/spirv_backend/src/lib.rs](crates/spirv_backend/src/lib.rs) and follow the examples.
 
+to use your new material alongside other materials like `IsomorphicConductor`, leverage the compound type wrappers
+in [`compound.rs`](./crates/cross/src/mat/compound.rs):
+`let my_sim = SpirvSim::<f32, DiscrMat2<IsomorphicConductor<f32>, MyMat>>::new(...)`
+
+if the compound wrappers seem complicated, it's because they have to be in order to be compatible with SPIR-V, which
+does not generally allow addresses to refer to more than one type.
+hence something like `enum { A(IsomorphicConductor<f32>), B(MyMat) }`
+has to instead be represented in memory like `(u8 /*discriminant*/, IsomorphicConductor<f32>, MyMat)`.
+in practice, we do some extra tricks to fold the discriminant into the other fields and reduce memory usage.
+
 as can be seen, the Material trait is fairly restrictive. its methods are immutable, and it doesn't even have access to the entire cell state (only the cell's M value, during `move_b_vec`). i'd be receptive to a PR or request that exposes more cell state or mutability: this is just an artifact of me tailoring this specifically to the class of materials i intended to use it for.
 
 ## What's in the Box