Add reference to original PR in Changelog
diff --git a/ChangeLog b/ChangeLog
index 7a710bd..13dda4d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,7 +6,7 @@
    * Fix possible memory leak in mbedtls_ssl_config_free().
      This can occur only if the user doesn't use mbedtls_ssl_conf_psk() and
      instead incorrectly manipulates conf->psk and/or conf->psk_identity
-     directly. Fix submitted by junyeonLEE.
+     directly. Found and fix submitted by junyeonLEE in #1220.
 
 = mbed TLS 2.7.x branch released 2018-xx-xx