Update README with duration flag information and example

[ldemailly]

Fixes #42

[Copilot]

Pull Request Overview

This PR adds documentation and example usage for duration flags in the CLI package. It demonstrates how to integrate the fortio.org/duration package with the existing CLI framework to support human-readable duration parsing.

• Added import and usage of the duration package to show duration flag support
• Updated example code to demonstrate duration flag creation and usage
• Added documentation explaining the integration with the duration package

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
sampleTool/main.go	Added duration flag example and updated log output to show parsed duration
go.mod	Added dependency on fortio.org/duration v1.0.1
README.md	Updated documentation with duration flag information and example usage

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