Documentation: mention bootstrap file for workers

[vikingjs]

It would have saved me a significant amount of time in development if I had known that I could supply a bootstrap file for workers. The documentation AND ERROR MESSAGES use "MUST be loadable by the composer class loader", which is not true. That guidance caused me to go through hoops when a simple one-line bootstrap file importing my autoloader would have sufficed.

Instead, all I had to do was:

	$workerFactory = new ContextWorkerFactory(__DIR__ . '/../Task/bootstrap.php');
	$this->workerPool ??= new ContextWorkerPool(50, $workerFactory);

I shouldn't have to stumble upon an important feature like that while reverse-engineering your library for another reason.