Enhance readme handling

[rindeal]

In my config file I wrote readme = "" and poetry install cmd then failed with a nondescript message that looked like this:

Installing the current project: my-project (0.1.0)
[Errno 21] Is a directory: '/home/runner/work/my-project/my-project'

This PR attempts to fix this.

• Updated documentation for changed code.
• Added tests for changed code.

Either add the tests yourselves or please advise on the best way to test the new code for 100% coverage.

[rindeal]

nice, thanks