Backfill/prepare CHANGELOG.md for imminent 0.4.0 release.

Author: eddyb

CHANGELOG.md

@@ -33,6 +33,27 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - ReleaseDate
 
+### Changed 🛠
+
+
+### Fixed 🩹
+- [PR#2](https://github.com/Rust-GPU/spirt/pull/2) included a combination of control-flow (restructurization) refactors and fixes:
+  - fixed rewrites of chains of `OpPhi`s (for e.g. Rust-GPU's non-minimal SSA form)
+  - isolated the `ControlRegion`s of unstructured SPIR-T CFGs from eachother
+    - i.e. removed the implicit dominance-based (SSA-style) "value visibility" semantics
+    - explicit (phi-like) propagation of *all* values across CFG edges required
+    - structured control-flow, however, retained its more flexible "value visibility"
+    - restructurization expected to "fuse" away the massively redundant dataflow
+  - 
+
+---
+
+### ⬆️ *above entries after repository move (to [`Rust-GPU/spirt`](https://github.com/Rust-GPU/spirt)*)
+*See also [the transition announcement blog post](https://rust-gpu.github.io/blog/transition-announcement/).*
+### ⬇️ *below entries before repository move (from [`EmbarkStudios/spirt`](https://github.com/EmbarkStudios/spirt))*
+
+---
+
 ### Changed 🛠
 - [PR#61](https://github.com/EmbarkStudios/spirt/pull/61) updated `SPIRV-Headers`
   to match Vulkan SDK 1.3.275