Add ChangeLog entry

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/ChangeLog.d/key-export.txt b/ChangeLog.d/key-export.txt
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index 0000000..5882d23
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+++ b/ChangeLog.d/key-export.txt
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+API changes
+   * mbedtls_ssl_conf_export_keys_ext_cb() has been removed.
+   * The signature of key export callbacks configured via
+     mbedtls_ssl_conf_export_keys_cb() has changed, and raw
+     keys and IVs are no longer exported. Further, callbacks
+     now receive an additional parameter indicating the type
+     of secret that's being exported, paving the way for the
+     larger number of secrets in TLS 1.3.