Add info to usage of --target

[joergdw]

Description

On usage I discovered that the relative paths are not realtive to the $PWD but rather to the directory specified by --source (which was different from $PWD in my case). I propose to state this explicitly.

Checklist

• Code compiles correctly
• Relevant tests were added (unit / contract / integration)
• Relevant logs were added
• Formatting and linting run locally successfully
• All tests pass
• UA review
• Design is documented
• Extended the README / documentation, if necessary
• Open source is approved

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