Improve the instructions for composer install in the readme#9

rpkoller wants to merge 2 commits intojoachim-n:2.0.xjoachim-n/ddev-drupal-core-dev:2.0.xfrom

rpkoller:20250315-rpkoller-composer-install-instructionsrpkoller/ddev-drupal-core-dev:20250315-rpkoller-composer-install-instructionsCopy head branch name to clipboard