add a caveats section

README.rst

@@ -96,6 +96,22 @@ You can install "pytest-run-parallel" via `pip`_ from `PyPI`_::
     $ pip install pytest-run-parallel
 
 
+Caveats
+-------
+
+Pytest itself is not thread-safe and it is not safe to share stateful pytest
+fixtures or marks between threads. Existing tests relying on setting up mutable
+state via a fixture will see the state shared between threads. Tests that
+dynamically set marks or share marks will also likely not be thread-safe. See
+the pytest documentation `for more detail <pytest-thread-safety>`_ and the
+community-maintained `free threaded Python porting guide
+<porting-guide-thread-safety>`_ for more detail about using pytest in a
+multithreaded context on the free-threaded build of Python.about thread safety
+in pytest.
+
+We suggest marking tests that are incompatible with this plugin's current design
+with ``@pytest.mark.thread_unsafe``.
+
 Usage
 -----
 
@@ -198,3 +214,5 @@ If you encounter any problems, please `file an issue`_ along with a detailed des
 .. _`pip`: https://pypi.org/project/pip/
 .. _`PyPI`: https://pypi.org/project
 .. _`PEP703`: https://peps.python.org/pep-0703/
+.. _`pytest-thread-safety`: https://docs.pytest.org/en/stable/explanation/flaky.html#thread-safety
+.. _`porting-guide-thread-safety`: https://py-free-threading.github.io/porting/#pytest-is-not-thread-safe