Update the version number to 1.3.20
diff --git a/ChangeLog b/ChangeLog
index b0d0860..1672bdf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS 1.3.20 released xxxx-xx-xx
+= mbed TLS 1.3.20 released 2017-06-21
 
 Security
    * Fixed unlimited overread of heap-based buffer in ssl_read().