[spirv-reader] Add README.md

Bug: tint:3
Change-Id: Ie711f7bdbb870c9f370ce777653884c5942a0622
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25260
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

src/reader/spirv/README.md

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+# SPIR-V Reader
+
+This component translates SPIR-V written for Vulkan into the Tint AST.
+
+The SPIR-V reader entry point is `tint::reader::spirv::Parser`, which
+implements the Reader interface in `tint::reader::Reader`.
+
+It's usable from the Tint command line:
+
+    # Dump the Tint AST after reading SPIR-V.
+    tint --dump-ast --parse-only a.spv
+
+    # Translate SPIR-V into WGSL.
+    tint --format wgsl a.spv
+
+## Validation
+
+The SPIR-V module must pass validation for the `SPV_ENV_WEBGPU_0` target
+environment in SPIRV-Tools.
+
+That set of rules is _experimental_ and was originally intended
+to constraint SPIR-V modules being ingested directly by the WebGPU API.
+Those rules are now too restrictive, because some amount of sanitization
+and normalization occurs during translation from SPIR-V to WGSL.
+The validation rules will be relaxed at some point TBD.
+
+Generally, validation of _functionality_ used will remain, e.g. WebGPU currently
+does not support `CullDistance` or subgroup operations.
+
+However, detailed rules about the form of SPIR-V can be relaxed, e.g. the
+requirement to use SPIR-V 1.3, and restrictive rules about statically unreachable
+code.
+
+## Feedback
+
+Please file issues at https://crbug.com/tint, and put `spirv-reader` in the issue title.
+
+Outstanding issues can be found by using the `spirv-reader` label in the Chromium project's
+bug tracker: https://bugs.chromium.org/p/tint/issues/list?q=label:spirv-reader