gh-115999: Note Python 3.14 free-threaded changes in What's New

Doc/whatsnew/3.14.rst

@@ -69,6 +69,7 @@ Summary -- release highlights
 * :ref:`PEP 761: Discontinuation of PGP signatures <whatsnew314-pep761>`
 * :ref:`A new type of interpreter  <whatsnew314-tail-call>`
 * :ref:`PEP 765: Disallow return/break/continue that exit a finally block <whatsnew314-pep765>`
+* :ref:`Free-threaded CPython <whatsnew314-free-threaded-cpython>`
 
 
 Incompatible changes
@@ -312,6 +313,24 @@ 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 mechanism is almost same
+as the default build, however in a free-threaded CPython build, each thread
+maintains a thread-local copy of specialized bytecode.
+
+With this change, the free-threaded CPython build achieves better performance,
+and is only 10-15% slower than the default build.
+
+This work was done by many contributors, Matt Page, Neil Schemenauer,
+Sam Gross, Thomas Wouters, Donghee Na, Kirill Podoprigora and Ken Jin.
+Meta also provided significant engineering resources for performance benchmarking and
+design of per thread specialized bytecode mechanisms.
+
 
 Other language changes
 ======================