Setup local development stack using DDEV

Updated README with instructions.

We use DDEV to work on this module. The DDEV configuration is ignored from git for two reasons:

1. It's unnecessary.
2. It locks in the Drupal version.

Instead, we'll use a script to set up the development environment. Fortunately, [ddev-drupal-contrib](https://github.com/ddev/ddev-drupal-contrib) makes this very easy for us. The steps mentioned there are scripted in `./setup_dev.sh` and some more. The script also lets you specify the Drupal version to use (corresponds to DDEV recipes: `drupal8`, `drupal9`, and `drupal10` as of right now). It also copies a settings.local.php and modifies settings.php to have it take effect.

Usage:

./setup_dev.sh drupal9

This will set up a DDEV site at the address https://taarikh-drupal9.ddev.site (if you have cloned this repo in a directory called `taarikh`). Now you can install the site and happy testing.

Automatically closed - issue fixed for 2 weeks with no activity.