Prepare Changelog for 1.3 branch development

This is meant to minimize/simplify merge conflict between topic branches.
diff --git a/ChangeLog b/ChangeLog
index f5a3867..87098c9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,15 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 1.3 branch
+
+Security
+
+Features
+
+Bugfix
+
+Changes
+
 = mbed TLS 1.3.10 released 2015-02-09
 Security
    * NULL pointer dereference in the buffer-based allocator when the buffer is