Provide information about "openssl dgst" -hmac option.

apps/dgst.c

@@ -229,6 +229,7 @@ int MAIN(int argc, char **argv)
 		BIO_printf(bio_err,"-keyform arg    key file format (PEM or ENGINE)\n");
 		BIO_printf(bio_err,"-signature file signature to verify\n");
 		BIO_printf(bio_err,"-binary         output in binary form\n");
+		BIO_printf(bio_err,"-hmac key       create hashed MAC with key\n");
 #ifndef OPENSSL_NO_ENGINE
 		BIO_printf(bio_err,"-engine e       use engine e, possibly a hardware device.\n");
 #endif