[tz] [PROPOSED] Mention RFC 8633 on leap smearing

[Paul Gilmartin]

On 2019-09-11, at 02:15:28, Paul Eggert wrote:
> 
> * tz-link.html: Cite RFC 8633 on leap smearing, and update some URLs.
> ---
I see that RFC 8633 cautions against public-facing servers' using
leap smearing, and NTP clients' using a mixture of smeared and
unsmeared.  I suppose the same should apply to quasi-NTP UT1
servers such as:
    https://www.nist.gov/pml/time-and-frequency-division/time-services/ut1-ntp-time-dissemination

-- gil