docs: add checkout step to source build instructions

[davebarnwell]

Summary

• add clone and checkout context to the README source build steps
• clarify how to build a specific tag or branch from a local checkout

Testing

• not run (documentation change only)

[Copilot]

Pull request overview

This documentation-only PR adds the git clone and cd steps to the "Build from source" section of the README, making the instructions self-contained for users who don't already have the repository checked out. It also includes a helpful commented hint about checking out a specific tag or branch.

Changes:

• Added git clone and cd commands to the "Build from source" code block in the README.
• Added a commented hint for checking out a specific release tag or branch.

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