Magento2 add product attribute programatically. Based on https://www.atwix.com/magento/adding-attribute-programatically-magento2/

InstallData.php

<?php namespace TimNeutkens\InstallProductAttributes\Setup;

use Magento\Catalog\Model\Product;
use Magento\Catalog\Model\ResourceModel\Eav\Attribute;
use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
    /**
     * EAV setup factory
     *
     * @var EavSetupFactory
     */
    private $eavSetupFactory;

    /**
     * Init
     *
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

        /**
         * Add attributes to the eav/attribute
         */
        $eavSetup->addAttribute(
            Product::ENTITY,
            'test_attribute',
            [
                'type'                     => 'text',
                'backend'                  => '',
                'frontend'                 => '',
                'label'                    => 'Extra information',
                'input'                    => '',
                'class'                    => '',
                'source'                   => '',
                'global'                   => Attribute::SCOPE_GLOBAL,
                'visible'                  => true,
                'required'                 => false,
                'user_defined'             => false,
                'default'                  => '',
                'searchable'               => false,
                'filterable'               => false,
                'comparable'               => false,
                'visible_on_front'         => true,
                'used_in_product_listing'  => true,
                'unique'                   => false,
                'apply_to'                 => '',
                'frontend_input'           => 'textarea',
                'is_html_allowed_on_front' => true,
                'is_wysiwyg_enabled'       => true,
            ]
        );
    }
}