Add changelog entry for TLS-Exporter feature

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>

ChangeLog.d/add-tls-exporter.txt

diff --git a/ChangeLog.d/add-tls-exporter.txt b/ChangeLog.d/add-tls-exporter.txt
new file mode 100644
index 0000000..c752a18
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+++ b/ChangeLog.d/add-tls-exporter.txt
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+Features:
+   * Add the function mbedtls_ssl_export_keying_material() which allows the
+     client and server to extract additional shared symmetric keys from an SSL
+     session, according to the TLS-Exporter specification in RFC 8446 and 5705.