Update ch01-03-hello-cargo.md

[Luxosaucer]

Added additional information about why using package managers are important so that new programmers have greater context on why you should use Cargo.

[chriskrycho]

Thanks for the suggestion, but I think this is covered—albeit more briefly—in the first paragraph of this section. We’re trying to avoid adding more text where possible, because the book is already huge. Thanks, though!