README: Mention support "async" keyword from Python 3.5.

README.md

@@ -22,11 +22,12 @@ WARNING: this project is in beta stage and is subject to changes of the
 code-base, including project-wide name changes and API changes.
 
 MicroPython implements the entire Python 3.4 syntax (including exceptions,
-"with", "yield from", etc.).  The following core datatypes are provided:
-str (including basic Unicode support), bytes, bytearray, tuple, list, dict,
-set, frozenset, array.array, collections.namedtuple, classes and instances.
-Builtin modules include sys, time, and struct.  Note that only subset of
-Python 3.4 functionality implemented for the data types and modules.
+"with", "yield from", etc., and additionally "async" keyword from Python 3.5).
+The following core datatypes are provided: str (including basic Unicode
+support), bytes, bytearray, tuple, list, dict, set, frozenset, array.array,
+collections.namedtuple, classes and instances. Builtin modules include sys,
+time, and struct.  Note that only subset of Python 3.4 functionality
+implemented for the data types and modules.
 
 See the repository www.github.com/micropython/pyboard for the Micro
 Python board, the officially supported reference electronic circuit board.