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diff --git a/README.md b/README.md
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+++ b/README.md
@@ -20,6 +20,7 @@ Date: Tue, 21 May 2013 04:31:30 GMT
 ```
 
 ## Requirement
+- mruby-io (https://github.com/iij/mruby-io)
 - system must have RFC3493 basic socket interface
 - and some POSIX API...