Enhancement: Use shell for fenced code blocks (#891)

Enhancement: Run friendsofphp/php-cs-fixer on GitHub Actions

Co-authored-by: Markus Staab <maggus.staab@googlemail.com>

Closes GH-559.

Enhancement: Add Makefile

Closes GH-621.

Fix: Command (#627)

Enhancement: Add CONTRIBUTING.md

Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>

Closes GH-615.