NEXT-21200 - Add docs for composer plugin requirements

[keulinho]

No description provided.

[what-the-diff]

• Update the shopware/core dependency to 6.5.*
• Add a new composer package as plugin dependency: "sebastian/exporter": "*"
• Overwrite executeComposerCommands() in our plugin base class and return true, so that shopware executes composer commands during installation of this plugin (composer require swag/basic-example)