Add "exec --last" to README.md

[euneuber]

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There is no hint about this feature beside the CHANGELOG and the commit.

[pbek]

Thank you! Hm, every command has parameters, plus the section where you added the line was generated by qc itself. I guess it would make more sense to just add a new paragraph Example usage with a bash code block with the full command with parameter and a comment what it does...