Add entry in ChangeLog

Add an entry in the ChangeLog, describing the fix.
diff --git a/ChangeLog b/ChangeLog
index 027a971..c28f806 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Add ecc extensions only if an ecc based ciphersuite is used.
+     Affects interoperability with BouncyCastle and other peers.
+     Raised by milenamil in #1157.
+
 = mbed TLS 2.11.0 branch released 2018-06-18
 
 Features