Update "Installation" instructions to include modern Git configuration location (.config/git)

[salcode]

Currently the line says

salcode-git/README.md

Line 91 in 7901e74

- Modify your default Git config file (`~/.gitconfig`) to include the config from this project. e.g. add

we should consider updating this to something like

- Modify your default Git config file, `~/.config/git/config` (or `~/.gitconfig`), to include the config from this project. e.g. add  

[salcode]

While ~/.config/git/config is the more modern location, in my tests when I set values in both ~/.config/git/config and ~/.gitconfig, the preferences in ~/.gitconfig take precedence.