composer require pluswerk/grumphp-config --dev

pluswerk/grumphp-config will add the required extra.grumphp.config-default-path automatically to your composer.json.

If pluswerk/grumphp-config should not edit your composer.json then you must add this:

{
  "extra": {
    "pluswerk/grumphp-config": {
      "auto-setting": false
    }
  }
}

Make a new grumphp.yml config file. You can put it in the root folder.

imports:
  - { resource: vendor/pluswerk/grumphp-config/grumphp.yml }


parameters:
  convention.xmllint_ignore_pattern:
    - "typo3conf/ext/extension/Resources/Private/Templates/List.xml"

There you can override some convention: