diff --git a/Lib/StringIO.py b/Lib/StringIO.py
index 04ffec9..8efd7d8 100644
--- a/Lib/StringIO.py
+++ b/Lib/StringIO.py
@@ -19,6 +19,8 @@
 
 Notes:
 - Using a real file is often faster (but less convenient).
+- There's also a much faster implementation in C, called cStringIO, but
+  it's not subclassable.
 - fileno() is left unimplemented so that code which uses it triggers
   an exception early.
 - Seeking far beyond EOF and then writing will insert real null