Remove code duplication in usage instructions

[gonzalo-bulnes]

The program welcome message and instructions make extensive use of the println! macro, because they run across multiple lines, and long lines are difficult to read in the code as well.

A raw string litteral could be used instead, that would:

• keep the message readable in the code
• keep the message formatting unchanged in the program output
• require only one println! : )

Note: I'd like to add some tests around this to make the refactoring easier, but I'll be more than happy to accept a contribution based on manual testing if I haven't added them when you pick it up!

[Bassetts]

I'd like to pick this up please 🙂