Add issue number to ChangeLog

Resolves #1122
diff --git a/ChangeLog b/ChangeLog
index 2f17367..14e0982 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,7 +4,7 @@
 
 Features
    * Allow comments in test data files.
-   * Add support for public keys encoded in PKCS#1 format.
+   * Add support for public keys encoded in PKCS#1 format. #1122
 
 Bugfix
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.