`fuelup <SUBCOMMAND> --help` should display more information about the command

# Summary

When looking at the help section of the `install` sub-command it does not tell me anything additional to what `fuelup --help` tells me for that sub-command. It actually shows me less info.
All it shows me is the usage - which at this point (with the number of sub-commands and their simplicity) is useless.

Fix: Add a `Description` section or something equivalent to expand on the 1 liner presented by just using `fuelup --help`
Note: How rustup does it.

# Screenshots

## fuelup

## rustup