pythongh-11599: Update whats news for free-threaded 3.14

Author: corona10

Doc/whatsnew/3.14.rst

@@ -68,6 +68,7 @@ Summary -- release highlights
 * :ref:`PEP 741: Python Configuration C API <whatsnew314-pep741>`
 * :ref:`PEP 761: Discontinuation of PGP signatures <whatsnew314-pep761>`
 * :ref:`A new type of interpreter  <whatsnew314-tail-call>`
+* :ref:`Free-threaded CPython <whatsnew314-free-threaded-cpython>`
 
 
 Incompatible changes
@@ -311,6 +312,24 @@ For further information on how to build Python, see
 (Contributed by Ken Jin in :gh:`128563`, with ideas on how to implement this
 in CPython by Mark Shannon, Garrett Gu, Haoran Xu, and Josh Haberman.)
 
+.. _whatsnew314-free-threaded-cpython:
+
+Free-threaded CPython
+---------------------
+
+Free-threaded CPython now supports :pep:`659` which is known
+as the specializing adaptive interpreter, the execution mechanisms is almost same
+as the default build, but at free-threaded CPython build, each thread maintains
+a thread-local copy of specialized bytecode.
+
+With this change, the free-threaded CPython build achieve better performance
+and 10-15% slower than the default build.
+
+This work was done by many contributors, Matt Page, Neil Schemenauer,
+Sam Gross, Thomas Wouters, Donghee Na and Kirill Podoprigora
+Meta also provided significant engineering resources for performance benchmark and
+design per thread specialized bytecode mechanisms.
+
 
 Other language changes
 ======================