Installation documentation

I was able to get it installed with composer using the following format:

{
    ....
    "repositories": [
        {
            "type": "composer",
            "url": "https://packagist.drupal-composer.org"
        },
        {
          "type": "package",
          "package": {
            "name": "phpoffice/phpspreadsheet",
            "version": "0.0",
            "dist": {
              "url": "https://github.com/PHPOffice/PhpSpreadsheet/archive/develop.zip",
              "type": "zip"
            }
          }
        }
    ],
    "require": {
        "composer/installers": "^1.0.20",
        "phpoffice/phpspreadsheet": "~0.0",
        "drupal/core": "8.2.0",
        "drupal/migrate_spreadsheet": "8.1.0-alpha2",
    },
    "autoload": {
        "psr-4": {
            "Drupal\\composer_manager\\Composer\\": "web/modules/contrib/composer_manager/src/Composer",
            "PhpOffice\\PhpSpreadsheet\\": "vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet"
        }
    }
    ....
}