Note for loading telegram drivers to receive attachments

[sasha-x]

It seems I can't reopen #69 , but info is actual and absent in main docs.

You need to manually load corresponding driver to receive attachments, for example:

DriverManager::loadDriver(\BotMan\Drivers\Telegram\TelegramPhotoDriver::class);

Otherwise it silently don't work.

Add it somewhere, plz.
Here: https://github.com/botman/docs/blob/master/driver-telegram.md
or here: https://github.com/botman/docs/blob/master/receiving-additional-content.md .

Should I make PR?