Add documentation for design meetings

[llvm-beanz]

This PR adds documentation and process outlines for the public design meetings that we will begin holding next week.

[damyanp]

LGTM

Have you considered adding new github project to track design meeting items?

[llvm-beanz]

Have you considered adding new github project to track design meeting items?

That's a great idea. We have the HLSL Language board, which we could restructure/reuse. I'll think about that, but suggestions welcome.