Add ChangeLog entry

Add ChangeLog entry describing the new key export feature.
diff --git a/ChangeLog b/ChangeLog
index 58ff147..e205835 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -61,6 +61,8 @@
    * Add MBEDTLS_REMOVE_3DES_CIPHERSUITES to allow removing 3DES ciphersuites
      from the default list (enabled by default). See
      https://sweet32.info/SWEET32_CCS16.pdf.
+   * Extend the  MBEDTLS_SSL_EXPORT_KEYS to export the handshake randbytes,
+     and the used tls-prf.
 
 API Changes
    * Add a new X.509 API call `mbedtls_x509_parse_der_nocopy()`.