The bundle is listed on packagist under 'avro/generator-bundle'. Just add it to your composer.json file.

        "require": {
            "avro/generator-bundle": "dev-master"
        }
    

Enable the bundle in the kernel:

        <?php
        // app/AppKernel.php

        if (in_array($this->getEnvironment(), array('dev', 'test'))) {
            ...
            $bundles[] = new Avro\GeneratorBundle\AvroGeneratorBundle();
        }
    

Now, update composer to download the bundle:

        $ php composer.phar update