Add a proposal to describe 202x and 202y (#178)

I've talked about 202x and 202y in passing but haven't really written
down my thoughts. This proposal is the start of writing down how I think
we should classify the two language versions.

proposals/0020-hlsl-202x-202y.md

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+
+# HLSL 202x and 202y
+
+* Proposal: [0020](0020-hlsl-202x-202y.md)
+* Author(s): [Chris Bieneman](https://github.com/llvm-beanz)
+* Sponsor: [Chris Bieneman](https://github.com/llvm-beanz)
+* Status: **Under Consideration**
+* Impacted Projects: DXC & Clang
+
+## Introduction
+
+This proposal seeks to provide a framework for defining the next two versions of
+the HLSL programming language. It seeks to define HLSL 202x as a bridge between
+DXC's implementation and Clang's implementation, and HLSL 202y as the next major
+feature release.
+
+## Motivation
+
+The HLSL compiler is undergoing a massive transition moving to Clang. Since
+the HLSL language is not formally specified and existing implementations
+disagree about fundamental behaviors, the new compiler will not be fully
+compatible with the previous compilers. This will create three conflicting
+implementations.
+
+This poses a challenge for users as they seek to migrate to Clang. Sources used
+with DXC may not be compatible with Clang by preventing the ability to switch to
+Clang piecemeal or being able to A/B test shaders.
+
+## Version Naming and Management
+
+HLSL versions names match the intended year of completion. Historically we
+missed with HLSL 2021 not completing the implementation and deployment in 2023.
+Due to schedule uncertainty and a desire to not repeat _that mistake_, future
+versions have placeholder names.
+
+The next language version is HLSL 202x. HLSL 202y will release after 202x. As we
+near the final release of 202x the release name will change to the target year.
+The same will follow for 202y.
+
+The expectation is that HLSL `x` is a placeholder integer value less than `y`.
+All features added to HLSL 202x will be implicitly inherited by HLSL 202y.
+
+## Proposed Solution
+
+This proposal adopts the development of two new language versions for HLSL in
+parallel. The proposal adopts a narrowly focused HLSL 202x which will be
+supported by both DXC and Clang, and a wider focused HLSL 202y feature release
+which will only be supported by Clang.
+
+### HLSL 202x
+
+HLSL 202x shall contain features that bridge compatibility between DXC and
+Clang. This shall be limited to cases where Clang's implementations do not match
+DXC and it is expected to cause potential disruption.
+
+Existing proposals that fall into this category are:
+* [Numeric Constants](/proposals/0003-numeric-constants.md)
+* [Conforming Literals](/proposals/0017-conforming-literals.md)
+
+HLSL 202x will not include features for all differences between DXC and Clang.
+Some of the
+[differences](https://clang.llvm.org/docs/HLSL/ExpectedDifferences.html), are
+not expected to have meaningful difference to users, or have reasonable source
+compatible workarounds. HLSL 202x is specifically for differences that do not
+have workarounds and may pose a barrier to adoption.
+
+### HLSL 202y
+
+HLSL 202y shall contain features that expand HLSL. This is all other outstanding
+feature proposals.
+
+
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