Add CODECHECK badge to README

[Robinlovelace]

A CODECHECK certificate has been issued for the paper "ClockBoard: A zoning system for urban analysis".

We need to add the CODECHECK "CODE WORKS" badge to the README.

Markdown snippet:
[![CODECHECK](https://codecheck.org.uk/img/codeworks-badge.svg)](https://doi.org/10.5281/zenodo.17059066)

Certificate URL: https://doi.org/10.5281/zenodo.17059066