Add missing plugin activation steps to Composer installation instructions

[Copilot]

The installation documentation was incomplete for Composer-based installations. The README only showed:

composer require frosh/tools

However, in Shopware 6, installing a plugin via Composer requires additional steps to register and activate the plugin. Without these steps, the bundle is not added to the Shopware application and users encounter the issue described where "the bundle is not activated for a vanilla Shopware 6.7 installation."

This PR adds the missing commands to both Packagist and Store installation sections:

composer require frosh/tools
bin/console plugin:refresh
bin/console plugin:install --activate FroshTools

The plugin:refresh command updates Shopware's plugin registry to recognize the newly installed plugin, and plugin:install --activate installs and activates it in one step.

These commands are already used elsewhere in the codebase (see SwagSecurityChecker.php) and follow standard Shopware 6 plugin installation practices.

Fixes #368.

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