Document Cython's freethreading_compatible directive

Author: lysnikolaou

docs/porting.md

@@ -22,17 +22,17 @@ Extension modules need to explicitly indicate they support running with the GIL
 disabled, otherwise a warning is printed and the GIL is re-enabled at runtime
 after importing a module that does not support the GIL. 
 
-!!! note
-
-    Currently it is not possible for extensions written in Cython to declare
-    they support running without the GIL. Work is under way to add support
-    (see [cython#6242](https://github.com/cython/cython/pull/6242)).
-
 C++ extension modules making use of `pybind11` can easily declare support for
 running with the GIL disabled via the
 [`gil_not_used`](https://pybind11.readthedocs.io/en/stable/reference.html#_CPPv4N7module_23create_extension_moduleEPKcPKcP10module_def16mod_gil_not_used)
 argument to `create_extension_module`.
 
+Starting with Cython 3.1.0 (only available via the nightly wheels), extension modules
+written in Cython can also do so using the
+[`freethreading_compatible`](https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#compiler-directives) compiler directive. It can be enabled either
+per module or globally by adding `-Xfreethreading_comparible=True` to the Cython
+arguments via the project's build system.
+
 C or C++ extension modules using multi-phase initialization can specify the
 [`Py_mod_gil`](https://docs.python.org/3.13/c-api/module.html#c.Py_mod_gil)
 module slot like this: