diff --git a/ext/stringio/README b/ext/stringio/README
deleted file mode 100644
index d03f37dfd3..0000000000
--- a/ext/stringio/README
+++ /dev/null
@@ -1,18 +0,0 @@
--*- rd -*-
-$Author$
-
-=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
diff --git a/ext/stringio/README.md b/ext/stringio/README.md
new file mode 100644
index 0000000000..94532ed688
--- /dev/null
+++ b/ext/stringio/README.md
@@ -0,0 +1,10 @@
+# StringIO
+
+Pseudo `IO` class from/to `String`.
+
+This library is based on MoonWolf version written in Ruby.  Thanks a lot.
+
+## Differences to `IO`
+
+* `fileno` raises `NotImplementedError`.
+* encoding conversion is not implemented, and ignored silently.