Update changelog

ChangeLog.rst

 5.2.0 - TBD
 -----------
 
-* Optimized format string parsing and argument processing
+* Optimized format string parsing and argument processing which resulted in up
+  to 5x speed up on long format strings and significant performance boost on
+  various benchmarks. For example, version 5.2 is 2.2x faster than 5.1 on
+  decimal integer formatting with ``format_to`` (macOS, clang-902.0.39.2):
+
+  ==================  =======  =======
+  Method              Time, s  Speedup
+  ==================  =======  =======
+  fmt::format 5.1      0.58
+  fmt::format 5.2      0.35     1.66
+  fmt::format_to 5.1   0.51
+  fmt::format_to 5.2   0.23     2.22
+  sprintf              0.71
+  ==================  =======  =======
 
 * Changed the ``fmt`` macro from opt-out to opt-in to prevent name collisions.
   To enable it define the ``FMT_STRING_ALIAS`` macro to 1 before including