Note the reordered fields in SSL structures

This is technically an API break according to the unwritten rules of API
compatibility for Mbed TLS 2.x. However, it is very unlikely to affect any
realistic application, with the possible exception of applications that
define a global constant of type mbedtls_ssl_config.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/semi-public-structure-fields.txt b/ChangeLog.d/semi-public-structure-fields.txt
new file mode 100644
index 0000000..802f8de
--- /dev/null
+++ b/ChangeLog.d/semi-public-structure-fields.txt
@@ -0,0 +1,5 @@
+API changes
+   * Some fields of mbedtls_ssl_session and mbedtls_ssl_config are in a
+     different order. This only affects applications that define such
+     structures directly or serialize them.
+