Warning "empty zet folder detected" is too simple

[tpwo]

It just looks for the README.md inside, if it is gone, then the warning is raised, even if there are additional files in the same folder.

When trying to clean this folder with pyzet clean nothing happens, as this command actually checks if the folder is empty, and proceeds only in such situation.

The solution is probably to add a new type of warning if there are additional files in the folder.