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

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

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