Improve the document for section build from source

[SteveLauC]

One can install this tool through:

$ cargo install --git https://github.com/K4rakara/freshfetch

And freshfetch will be put under:

$ which freshfetch
/home/$USER/.cargo/bin/freshfetch

To update it, just run the install command again. Or one can use cargo-update:

$ cargo install-update -g freshfetch

To delete it:

$ cargo uninstall freshfetch