Adds recipe for leerzeichen mode

A brief summary of what the package does: Introduces a minimalistic minor mode to make whitespace characters visible without font-lock.

A direct link to the package repository: https://github.com/fgeller/leerzeichen.el/

Your association with the package: Maintainer.

I installed and tested the recipe locally via the sandbox.

Please let me know if you require any further information. Thanks!