Add notes about py3.8/asyncio/proactor to readme

Author: altendky

README.rst

@@ -18,6 +18,47 @@ which uses the twisted framework. test functions can return Deferred
 objects and pytest will wait for their completion with this plugin.
 
 
+NOTICE: Python 3.8 with asyncio support
+=======================================
+
+In Python 3.8, asyncio changed the default loop implementation to use
+their proactor.  The proactor does not implement some methods used by
+Twisted's asyncio support.  The result is a ``NotImplementedError``
+exception such as below.
+
+.. code-block:: pytb
+
+    <snip>
+      File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 320, in install
+        reactor = AsyncioSelectorReactor(eventloop)
+      File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 69, in __init__
+        super().__init__()
+      File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\base.py", line 571, in __init__
+        self.installWaker()
+      File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\posixbase.py", line 286, in installWaker
+        self.addReader(self.waker)
+      File "c:\projects\pytest-twisted\.tox\py38-asyncioreactor\lib\site-packages\twisted\internet\asyncioreactor.py", line 151, in addReader
+        self._asyncioEventloop.add_reader(fd, callWithLogger, reader,
+      File "C:\Python38-x64\Lib\asyncio\events.py", line 501, in add_reader
+        raise NotImplementedError
+    NotImplementedError
+
+The previous default, the selector loop, still works but you have to
+explicitly set it.  ``pytest_twisted.use_asyncio_selector_if_required()``
+is provided to help in choosing the selector loop.  It must be called
+early so the following `conftest.py` is suggested.
+
+.. code-block:: python3
+
+    import pytest
+    import pytest_twisted
+
+
+    @pytest.hookimpl(tryfirst=True)
+    def pytest_configure(config):
+        pytest_twisted.use_asyncio_selector_if_required(config=config)
+
+
 Python 2 support plans
 ======================