Add warning regarding fork and asyncio (#1519)

Author: cburgdorf

trinity/utils/mp.py

@@ -13,6 +13,8 @@
 from types import TracebackType
 
 
+# WARNING: Think twice before experimenting with `fork`. The `fork` method does not work well with
+# asyncio yet. This might change with Python 3.8 (See https://bugs.python.org/issue22087#msg318140)
 MP_CONTEXT = os.environ.get('TRINITY_MP_CONTEXT', 'spawn')