Merge branch 'issue64' into 'master'

Add an important note about Python < 3.7

Closes python#64

See merge request python-devs/importlib_resources!65

Author: warsaw

importlib_resources/docs/using.rst

@@ -150,6 +150,27 @@ manager that you can use in a ``with``-statement::
 You can use all the standard :py:mod:`contextlib` APIs to manage this context
 manager.
 
+.. attention::
+
+   There is an odd interaction with Python 3.4, 3.5, and 3.6 regarding adding
+   zip or wheel file paths to ``sys.path``.  Due to limitations in `zipimport
+   <https://docs.python.org/3/library/zipimport.html>`_, which can't be
+   changed without breaking backward compatibility, you **must** use an
+   absolute path to the zip/wheel file.  If you use a relative path, you will
+   not be able to find resources inside these zip files.  E.g.:
+
+   **No**::
+
+       sys.path.append('relative/path/to/foo.whl')
+       resource_bytes('foo/data.dat')  # This will fail!
+
+   **Yes**::
+
+       sys.path.append(os.path.abspath('relative/path/to/foo.whl'))
+       resource_bytes('foo/data.dat')
+
+Both relative and absolute paths work for Python 3.7 and newer.
+
 
 .. rubric:: Footnotes