Add a warning about the xfail support state

Author: webknjaz

src/pytest_forked/__init__.py

@@ -1,4 +1,6 @@
 import os
+import warnings
+
 import py
 # we know this bit is bad, but we cant help it with the current pytest setup
 from _pytest import runner
@@ -102,5 +104,10 @@ def report_process_crash(item, result):
             crash_info=info,
         )
     )
+    warnings.warn(
+        'pytest-forked xfail support is incomplete at the moment and may '
+        'output a misleading reason message',
+        RuntimeWarning,
+    )
 
     return rep