[TASK] Document PHPCS dependencies install step (#617)
[TASK] Document PHPCS dependencies install step (#617)

Following #576 and #589, the dependencies for PHP_CodeSniffer now require
PHP 7.0, but cannot be included in `composer.json` while PHP 5.x is still
supported, so an extra Composer installation step is now required.
jjriv/emogrifier,MyIntervals/emogrifier