add initial documentation for spirv_to_llvm

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+# spirv_to_llvm library
+
+## `spirv_to_llvm/spirv_to_llvm.h`
+
+### `spirv_to_llvm::Type_descriptor`
+Type representing a SPIR-V Type. Does custom type layout instead of letting LLVM layout types to avoid creating types that need a stricter alignment than `::operator new` provides.  
+Members:
+- `decorations`: the SPIR-V decorations applied to this type.
+- `get_or_make_type`: returns the LLVM type that `this` translates to, generating it if needed. Also returns the alignment used to store the returned LLVM type.
+- `visit`
+- `Recursion_checker`: helper type used to prevent infinite recursion when generating LLVM types.  
+See `spirv_to_llvm::Struct_type_descriptor::get_or_make_type` to see how to use it.
+- `Recursion_checker_state`: state used by `Recursion_checker`.
+
+### `spirv_to_llvm::Simple_type_descriptor`
+Wrap a fundamental type (not based on any other SPIR-V type).
+
+### `spirv_to_llvm::Vector_type_descriptor`
+Wrap a vector type.
+
+### `spirv_to_llvm::Matrix_type_descriptor`
+Wrap a matrix type.
+
+### `spirv_to_llvm::Array_type_descriptor`
+Wrap an array type.
+
+### `spirv_to_llvm::Pointer_type_descriptor`
+Wrap a pointer type. If `base == nullptr`, then `this` is a forward declaration.
+
+### `spirv_to_llvm::Function_type_descriptor`
+Wrap a function type.
+
+### `spirv_to_llvm::Struct_type_descriptor`
+Wrap a struct type.  
+This can be in one of two states:  
+- Incomplete: The default state upon construction. Members can be added in this state.
+- Complete: The state transitioned to by calling `get_members(true)` or `get_or_make_type()`. Members can no longer be added.
+
+### `spirv_to_llvm::Constant_descriptor`
+A SPIR-V constant.  
+Members:
+- `get_or_make_value`: returns the LLVM value that `this` translates to, generating it if needed.
+
+### `spirv_to_llvm::Simple_constant_descriptor`
+A simple SPIR-V constant. Wraps a LLVM value.
+
+### `spirv_to_llvm::Converted_module`
+The results of converting a SPIR-V shader from SPIR-V to LLVM IR.
+
+### `spirv_to_llvm::Jit_symbol_resolver`
+Resolve built-in symbols for the JIT. This is where to add new built-in functions that are called by LLVM IR.
+
+### `spirv_to_llvm::spirv_to_llvm`
+Convert the provided SPIR-V shader to LLVM IR. Throws `spirv::Parser_error` on error.