Document DXC handling of resources in global non-static structs (#369)

This initial proposal documents the way DXC handles resources and resource arrays inside user-defined structs and classes which are declared as non-static at the shader global scope (the default `cbuffer` context `$Globals`).

It covers:
- single resource in a struct
- resource array in a struct
- arrays of structs that contain resources
- multiple resource kinds within the same struct
- resources in base classes
- static vs. dynamic indexing
- implicit binding of resources and resource arrays in structs
- error cases

Part 1 of #175

Author: hekota