Prompt Customization instructions for starship.rs

[david-alpert-os]

Is your feature request related to a problem? Please describe.
When using Starship.rs I would like to customize my Starship prompt and add the git mob status to the command line for visibility.

Additional context
This snippet seems to work

# ~/.config/starship.toml

[custom.git_mob_status]
command = 'if [[ "$(git mob-print --initials 2>/dev/null)" == "" ]]; then echo "(solo)"; else echo "($(git mob-print --initials))"; fi'
when = ''' command -v git-mob '''
format = '[$output]($style) '

Documentation on custom additions to the command prompt are here
https://starship.rs/config/#custom-commands

Observations:

• the if/then syntax is intended to show "solo" when the list of mob initials is empty. can be customized further to show an empty string instead.

Let's add this sample into the repo's README.md documentation with the other shell customization scripts.