Add a gitignore for working with DDEV

Added the gitignore.

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

Thanks for maintaining this great module!

I raised this issue in the DDEV Drupal Contrib issue queue, because I think the add-on should take care of this instead of the individual modules, see Add .gitignore file, excluding vendor, web, .ddev, .editorconfig, etc. by default #25.