update changelog

Author: Mark McCaskey

CHANGELOG.md

@@ -5,6 +5,7 @@ All PRs to the Wasmer repository must add to this file.
 Blocks of changes will separated by version increments.
 
 ## **[Unreleased]**
+- [#579](https://github.com/wasmerio/wasmer/pull/579) Fix bug in caching with LLVM and Singlepass backends.  Add `default-backend-singlepass`, `default-backend-llvm`, and `default-backend-cranelift` features to `wasmer-runtime` to control the `default_compiler()` function (this is a breaking change).  Add `compiler_for_backend` function in `wasmer-runtime`
 - [#542](https://github.com/wasmerio/wasmer/pull/542) Add SIMD support to wasmer and implement it in the LLVM backend only.
 
 ## 0.5.7