Clarify "b" mode under Unix.
diff --git a/Doc/tutorial/inputoutput.rst b/Doc/tutorial/inputoutput.rst
index d3b912a..f9eec97 100644
--- a/Doc/tutorial/inputoutput.rst
+++ b/Doc/tutorial/inputoutput.rst
@@ -200,7 +200,8 @@
written. This behind-the-scenes modification to file data is fine for ASCII
text files, but it'll corrupt binary data like that in :file:`JPEG` or
:file:`EXE` files. Be very careful to use binary mode when reading and writing
-such files.
+such files. On Unix, it doesn't hurt to append a ``'b'`` to the mode, so
+you can use it platform-independently for all binary files.
.. _tut-filemethods: