[nifragos]

Summary

• Adds CHANGELOG.md covering 1.0.0 through 1.2.0, including the two observable behavior notes in 1.2 (duplicates skipped; exceptions surface through the Task).
• Adds a callout under the README title pointing at the new Upgrading to 1.2 section. That section covers: bumping the package, checking the behavior notes, turning on validation and fixing what it reports, opting into telemetry, and replacing a hand-written Blazor Server scoping sender.

Docs only; no code changes. The README inside the NuGet package updates with the next package release.