asyncio doc: begin with warnings on asyncio traps

Doc/library/asyncio.rst

@@ -46,6 +46,11 @@ Here is a more detailed list of the package contents:
   you absolutely, positively have to use a library that makes blocking
   I/O calls.
 
+Asynchronous programming is more complex than classical "sequential"
+programming: see the :ref:`Develop with asyncio <asyncio-dev>` page which lists
+common traps and explains how to avoid them. :ref:`Enable the debug mode
+<asyncio-debug-mode>` during development to detect common issues.
+
 Table of contents:
 
 .. toctree::