readme: remove unnecessary pre tags

This also fixes the indentation of the code block.
zpm: consistenty use "fp" for opened files
Add whitespace around copyright comment

I think it separates it more nicely from the rest of the code.
readme: sync usage
Merge pull request #6 from mgeisler/small-fixes

Small fixes
Merge pull request #5 from mgeisler/readme

readme: remove unnecessary pre tags and update usage doc