ChangeLog.d/ecp_write_key.txt

Features
   * The new function mbedtls_ecp_write_key_ext() is similar to
     mbedtls_ecp_write_key(), but can be used without separately calculating
     the output length.

New deprecations
   * mbedtls_ecp_write_key() is deprecated in favor of
     mbedtls_ecp_write_key_ext().