[ffigen] Mention ObjectiveC support in changelog (#366)

pkgs/ffigen/CHANGELOG.md

diff --git a/pkgs/ffigen/CHANGELOG.md b/pkgs/ffigen/CHANGELOG.md
index ccb4635..6cae5fb 100644
--- a/pkgs/ffigen/CHANGELOG.md
+++ b/pkgs/ffigen/CHANGELOG.md
@@ -1,6 +1,8 @@
 ## 5.0.0
 
 - Stable release targeting Dart 2.17, supporting ABI-specific integer types.
+- _EXPERIMENTAL_ support for ObjectiveC on MacOS hosts. The API and output
+  might change at any point. Feel free to report bugs if encountered.
 
 # 5.0.0-dev.1
 - Fixed invalid default dart types being generated for `size_t` and `wchar_t`.