README: add note about timeline of py2.py3 support

as discussed in #765 (comment)

README.rst

@@ -18,6 +18,13 @@ Installation
 FontTools requires `Python <http://www.python.org/download/>`__ 2.7, 3.4
 or later.
 
+**NOTE** After January 1 2019, until no later than June 30 2019, the support
+for *Python 2.7* will be limited to only bug fixes, and no new features will
+be added to the ``py27`` branch. The upcoming FontTools 4.x series will require
+*Python 3.5* or above. You can read more `here <https://python3statement.org>`__
+and `here <https://github.com/fonttools/fonttools/issues/765>`__ for the
+reasons behind this decision.
+
 The package is listed in the Python Package Index (PyPI), so you can
 install it with `pip <https://pip.pypa.io>`__: