Update TLS 1.3 documentation and add change log

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/ChangeLog.d/tls13-server-version-negotiation.txt b/ChangeLog.d/tls13-server-version-negotiation.txt
new file mode 100644
index 0000000..989018b
--- /dev/null
+++ b/ChangeLog.d/tls13-server-version-negotiation.txt
@@ -0,0 +1,5 @@
+Features
+   * Add support for server-side TLS version negotiation. If both TLS 1.2 and
+     TLS 1.3 protocols are enabled, the TLS server now selects TLS 1.2 or
+     TLS 1.3 depending on the capabilities and preferences of TLS clients.
+     Fixes #6867.