Document the plan for how resource attributes map to DXIL and SPIR-V

[bogner]

Information on the current plan for how we're using attributes on the resource type in the clang AST to translate to DXIL is scattered about several different issues and various meeting notes, which can make it hard to find. Furthermore, discussion on the SPIR-V side of things has been fairly hand-wavy so far even though it does affect the design.

We need to write a proposal that encapsulates the decisions so far, and that discusses how the attributes will be used to drive code generation in both DXIL and SPIR-V. This work has some overlap with #42, but with a slightly different scope and focus. Once we've written this proposal we should take a look to see if it makes sense to combine the two documents.

[bogner]

Playground for how dxc represents various resource types in dxil and spir-v: https://hlsl.godbolt.org/z/8MsGMvq7c