Expand the README upgrade note into a guide covering 1.4.1 to 1.6

[ademar]

Summary

The README's "Upgrading to 1.5" section becomes an Upgrading guide, newest release first:

• 1.6: dates are returned in UTC.
  • The RFC sections behind the change, and the affected properties (DomainRenewResponse.ExDate and the four Nominet dates).
  • How unmarked values are treated.
  • A code sample for displaying (ToLocalTime()) and comparing (DateTime.UtcNow) the values.
  • Notes that string date properties are unchanged, and that DomainRenew now accepts an exDate directly.
• 1.5: server certificates are validated. The existing section, unchanged apart from its heading.
• 1.4.1: TLS version chosen by the operating system. Callers need to rebuild to pick up the new default, since C# compiles default values into call sites. Drop any explicit SslProtocols.Tls.

Testing

I ran the 1.6 code sample against 1.6.0 with TZ=Asia/Tokyo:

• ExDate is 2005-04-03T22:00:00Z with DateTimeKind.Utc, and ToLocalTime() gives 2005-04-04T07:00:00+09:00.
• DomainRenew given 2026-04-03T22:00:00.0Z sends curExpDate 2026-04-03.

The README is packed into the NuGet package, so nuget.org will show this guide from the next release.

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