diff --git a/md5.md b/md5.md
index fe850e3..33b7ca6 100644
--- a/md5.md
+++ b/md5.md
@@ -16,6 +16,7 @@ A ffi binding of the popular [MD5 implementation][md5 lib] by Alexander Peslyak.
 `md5.digest() -> digest`     \          Get a function that can consume multiple
 `digest(s[, #s])`            \          data chunks until called with no arguments to
 `digest() -> s`                         return the final hash.
+`md5.hmac(s, key) -> s`                 compute the HMAC-MD5 of a string.
 --------------------------------------- ---------------------------------------
 
 __NOTE__: All functions return the binary representation of the hash.