Add doc about the configuration file

[AngeCyp]

Add doc to explain how to pass a config file path to the binary

[titouanj]

The help flag returns:

--config <CONFIG>
          Path to the config file
          
          [default: alumet-config.toml]

Isn't it clear enough?

[AngeCyp]

it's just to have a better doc, but yes you're rigth, the line is clear enough

[titouanj]

And also, I think that this line should be added (if we want to) to the user-book not the dev-book