Add information about --generate-secret.

[domol]

When I tried to run that, I haven't noticed this flag, so it may be easier for the new people to add this information to the readme.

I have asked about this on Matrix, and was to asked add that :)

[thomaseizinger]

Thanks!

In general, --help is always a good starting point for learning a new commandline tool and contrary to documentation like this, doesn't get as easily outdated :)