Add Changelog entry for #767

Add Changelog entry for fix for alternative header/footers in CSR's.
diff --git a/ChangeLog b/ChangeLog
index 569bc79..3fb9295 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Changes
+   * Add support for alternative CSR headers, as used by Microsoft and defined
+     in RFC 7468. Found by Michael Ernst. Fixes #767.
+
 = mbed TLS 2.7.8 branch released 2018-11-30
 
 Security