Add a contributing guide that explains signed commits - #231

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: korelstar/phplint-problem-matcher@cb2b753750ec7bf13a7cde0a476df8c5605bdfb1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/