add README and CHANGELOG entries

CHANGELOG.md

@@ -6,6 +6,13 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Changed
+
+ - Refactored dependencies of experimental "specialization" feature into
+   default_fn! macro to reduce code duplication and complexity. Currently
+   only needed for non-functional SIMD feature.
+ - Refactored SIMD code into separate source files. See README.md for details.
+
 ### Added
 
  - Add `VectorN::zip` and `PointN::zip`

README.md

@@ -57,6 +57,13 @@ and `v.zy()` produces a
 ```rust
 Vector2 { x: 3.0, y: 2.0 }
 ```
+### SIMD optimizations
+
+The current SIMD support depends on the deprecated "simd" package as well
+as the unstable "specialization" feature. To build this code, a pre-1.33 nightly
+build of Rust is required, e.g. 2019-01-01-nightly. Though the code is not
+useful in its present form, it has some worth preserving as starting point
+for a future migration (see https://github.com/rustgd/cgmath/issues/490).
 
 ## Limitations