diff --git a/ext/stringio/README b/ext/stringio/README
new file mode 100644
index 0000000000..c4031f7e97
--- /dev/null
+++ b/ext/stringio/README
@@ -0,0 +1,19 @@
+-*- rd -*-
+$Author$
+$Date$
+
+=begin
+
+= StringIO
+Pseudo (({IO})) class from/to (({String})).
+
+This library is based on MoonWolf version written in Ruby.  Thanks a lot.
+
+= Differences to (({IO}))
+
+* not implemented: (({fcntl})), (({reopen})).
+* (({fileno})) returns nil.
+* (({pos=})) returns new position, not 0.
+* (({ungetc})) does nothing at start of the string. 
+
+=end