ext/stringio/README.md: update [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55326 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/stringio/README

@@ -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

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.