Out of date chunks in documentation

[Hal Murray]

We need a mechanism to review/update things occasionally and a list of things 
that need occasional review.

Here is a starter:
  https://www.ntpsec.org/supported-platforms.html
Under test status, it says:
  Fedora 26 and 25 (i686, x86_64)
and lots more.

We could fix most of that by replacing specific version numbers by something 
like "supported versions" and maybe mentioning that many distros have a policy 
of supporting the current release and the previous release.

-- 
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