Add ChangeLog entry

Author: eregon

CHANGELOG.md

@@ -21,6 +21,7 @@ Compatibility:
 
 Performance:
 
+* Change the `Hash` representation from traditional buckets to a "compact hash table" for improved locality, performance and memory footprint (#3172, @moste00).
 * Optimize calls with `ruby2_keywords` forwarding by deciding it per call site instead of per callee thanks to [my fix in CRuby 3.2](https://bugs.ruby-lang.org/issues/18625) (@eregon).
 * Optimize feature loading when require is called with an absolute path to a .rb file (@rwstauner).