Clarify documentation

The first line of the documentation, the part "[...] captures sent e-mails in development mode [...]" is misleading. Mails are always captured, just not shown in production.

I don't mind leaving MailPanel out in config.local on a production machine. Merely from this documentation I wrongly assumed this would let mails pass normally in procution mode.

Thanks @JanTvrdik , great component!