Update changelog

Author: vitaut

ChangeLog.md

@@ -2,7 +2,18 @@
 
 - Optimized the default floating point formatting
   (https://github.com/fmtlib/fmt/issues/3675,
-  https://github.com/fmtlib/fmt/issues/4516).
+  https://github.com/fmtlib/fmt/issues/4516). In particular, formatting a
+  `double` with format string compilation into a stack allocated buffer is
+  more than 60% faster in version 12.0 compared to 11.2 according to
+  [dtoa-benchmark](https://github.com/fmtlib/dtoa-benchmark):
+
+  ```
+  Function	Time (ns)  Speedup
+  fmt11        34.471    1.00x
+  fmt12	       21.000    1.64x
+  ```
+
+  <img width="766" height="609" src="https://github.com/user-attachments/assets/d7d768ad-7543-468c-b0bb-449abf73b31b" />
 
 - Added `constexpr` support to `fmt::format`. For example: