Add note that this library requires python3.3 or newer

You found me out for running python 3.2. The "yield from" statement used in this library was introduced in Python 3.3 [1], which means the library will only run on that or newer right now. This pull request adds a note on this to the README file.

[1] https://docs.python.org/3/reference/expressions.html#yieldexpr