changelog: Explain possible performance changes

Author: chfast

CHANGELOG.md

@@ -5,7 +5,7 @@ Documentation of all notable changes to the **Fizzy** project.
 The format is based on [Keep a Changelog],
 and this project adheres to [Semantic Versioning].
 
-## [0.7.0] - unreleased
+## [0.7.0] — unreleased
 
 With this release we aim to provide a much improved C and Rust API, including a clear separation of i32 and i64 types. 
 Fizzy passes all of the official WebAssembly 1.0 tests. We are maintaining the WebAssembly 1.0 test suite
@@ -15,6 +15,10 @@ Fizzy release [the snapshot from 2021-02-12](https://github.com/wasmx/wasm-spec/
 - 1009 of 1009 validation tests,
 - 499 skipped due to testing text format parser.
 
+There are no performance changes expected nor observed in this release. 
+With one exception: the `i32` member added to the `Value` union causes machine code being generated with different layout. This may impact the execution performance depending on the compiler and build configuration.
+In some isolated cases differences up to ±30% were observed.
+
 
 ### Added