Add changeLog entry

Add changeLog entry describing the new `mbedtls_ssl_tls_prf()` API.
diff --git a/ChangeLog b/ChangeLog
index e205835..f01b870 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -63,6 +63,7 @@
      https://sweet32.info/SWEET32_CCS16.pdf.
    * Extend the  MBEDTLS_SSL_EXPORT_KEYS to export the handshake randbytes,
      and the used tls-prf.
+   * Add public API for tls-prf function, according to requested enum.
 
 API Changes
    * Add a new X.509 API call `mbedtls_x509_parse_der_nocopy()`.