Point towards syntax of (aliases) file table for new users.

[stellarpower]

#684

[stellarpower]

A reference may want to be added in other locations too, I figured the example config file was probably going to be spotted by anyone who had questions on how to use it.

I also had mine without the colon, i.e. alias@localhost real-recipient@localhost, and this was parsed fine as the mapping "alias@localhost real-recipient@localhost" => "", which should perhaps be a parse error for the context of address rewriting and throw an error, or at least a warning.