[naga] Add a review checklist.

naga/REVIEW-CHECKLIST.md

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+# Naga Review Checklist
+
+This is a collection of notes on things to watch out for when
+reviewing pull requests submitted to Naga.
+
+Unfortunately, there are a few parts of Naga where certain knowledge,
+requirements, or concerns are spread out across several areas of the
+code. This makes mistakes easier, as it is more likely for a
+contributor to fix one spot, but neglect others.
+
+Ideally, one fixes this kind of problem by refactoring the code so
+that requirements are easier to identify with local reasoning. We do
+this regularly. But when such projects are too large to undertake
+immediately, we can use review checklists as a temporary workaround
+to ensure that all the different spots do get addressed.
+
+## General
+
+If your change iterates over a collection, did you ensure the order of
+iteration was deterministic? Using `HashMap` and `HashSet` is fine, as
+long as you don't iterate over it.
+
+## IR changes
+
+If your change adds or removes `Handle`s from the IR:
+- Did you update handle validation in `valid::handles`?
+- Did you update the compactor in `compact`?
+- Did you update `back::pipeline_constants::adjust_expr`?
+
+If your change adds a new operation:
+- Did you update the typifier in `proc::typifier`?
+- Did you update the validator in `valid::expression`?
+- If the operation can be used in constant expressions, did you
+  update the constant evaluator in `proc::constant_evaluator`?
+
+## Backend changes
+
+- If your change introduces any new identifiers, how did you ensure
+  they won't conflict with the users' identifiers? (This is usually
+  not relevant to the SPIR-V backend.)
+  - Did you use the `Namer` generate a fresh identifier?
+  - Did you register the identifier as a reserved word with the the `Namer`?
+  - Did you use a reserved prefix registered with the `Namer`?
+
+## WGSL Extensions
+
+If you added a new feature to WGSL that is not covered by of the WebGPU specification:
+
+- Did you add a `Capability` flag for it?
+- Did you document the feature fully in that flag's doc comment?
+- Did you ensure the validator rejects programs that use the feature?