Add notes about 3.14 changes to release notes

docs/source/changelog.rst

@@ -124,6 +124,18 @@ Added
 
 * Add Python 3.14 to test matrix and project classifiers.
   (`#751`_, `@LincolnPuzey`_)
+  Things to be aware of when upgrading to Python 3.14:
+
+  * Python 3.14 `changes`_ the default multiprocessing `start method`_ to `forkserver`_ on some platforms.
+    Dramatiq Workers use multiprocessing and will be effected by this change.
+    ``forkserver`` should be less bug-prone than the old default ``fork``.
+    However, if you run into weird issues, using the existing ``--use-spawn`` flag when starting Dramatiq to
+    set the start method to ``spawn``, might solve them.
+  * The free-threaded build of Python is now `officially supported`_ by Python.
+    Dramatiq is not yet unit-tested with free-threaded Python, but we hope to do so soon.
+    In the meantime, if you have any success or problems running Dramatiq with free-threaded Python,
+    we would love to hear about it.
+
 * Added type annotations for the external API of the |Worker| and |Broker| classes.
   (`#727`_, `#731`_, `#744`_, `@jenstroeger`_)
 * Added type annotations for the external API of the |Middleware| class and its subclasses.
@@ -134,6 +146,10 @@ Added
 * Added ``dramatiq_worker_timeout`` environment variable.
   (`#773`_, `@ksoviero-zengrc`_)
 
+.. _changes: https://docs.python.org/3.14/whatsnew/3.14.html#whatsnew314-multiprocessing-start-method
+.. _start method: https://docs.python.org/3.14/library/multiprocessing.html#multiprocessing-start-methods
+.. _forkserver: https://docs.python.org/3.14/library/multiprocessing.html#multiprocessing-start-method-forkserver
+.. _officially supported: https://docs.python.org/3.14/whatsnew/3.14.html#free-threaded-python-is-officially-supported
 
 .. _#751: https://github.com/Bogdanp/dramatiq/pull/751
 .. _#727: https://github.com/Bogdanp/dramatiq/issues/727

dramatiq/cli.py

@@ -221,10 +221,11 @@ def make_argument_parser():
         help="write all logs to a file (default: sys.stderr)",
     )
     parser.add_argument("--skip-logging", action="store_true", help="do not call logging.basicConfig()")
+    _unix_default = "fork" if sys.version_info < (3, 14) else "forkserver"
     parser.add_argument(
         "--use-spawn",
         action="store_true",
-        help="start processes by spawning (default: fork on unix, spawn on windows)",
+        help=f"start processes by spawning (default: {_unix_default} on unix, spawn on Windows and macOS)",
     )
     parser.add_argument(
         "--fork-function",