Add note to usage docs referring to PEP 427 directive discouraging use of wheels on sys.path. Fixes #52.

Author: jaraco

importlib_metadata/docs/using.rst

@@ -22,6 +22,13 @@ By default, package metadata can live on the file system or in wheels on
 ``sys.path``.  Through an extension mechanism, the metadata can live almost
 anywhere.
 
+.. note::  Although this package supports loading metadata from wheels
+   on ``sys.path``, that support is provisional and does not serve to
+   contravene the `PEP 427 directive
+   <https://www.python.org/dev/peps/pep-0427/#is-it-possible-to-import-python-code-directly-from-a-wheel-file>`_,
+   which states that relying on this format is discouraged, and use is
+   at your own risk.
+
 
 Overview
 ========