asyncio: Document version constraints

Showing the version limits for the example and the module. There are no
explicit sys_version checks in place to give a better error message
because a Python < 3.5 would fail with an ImportError at the mpd.asyncio
module or the example code before it could execute that check.

Author: chrysn

examples/summary.txt

@@ -22,3 +22,6 @@ ExampleStats  Get general information of your MPD server  2009-09-12
 ExampleStickers A command-line client for manipulating and querying stickers
 2010-12-18
 Examples  Some example scripts to show how to play with python-mpd  2010-12-18
+
+The asyncio_example.py shows how MPD can be used with the asyncio idioms; it
+requires at least Python 3.5 to run.

mpd/asyncio.py

@@ -14,6 +14,9 @@
 pushers).
 
 Command lists are currently not supported.
+
+
+This module requires Python 3.5.2 or later to run.
 """
 
 import asyncio