Add a chapter on custom formatting to the stagin area

Using cities like in json :-)
I have no idea where this could go in the structure since traits are a bit too advanced a topic for the 2nd chapter...

I usually format struct initializers as:

`City { name: "Vancouver", lat: 49.25, lon: -123.1 },`

The optional comma before a `]`, `)` is only useful (for diff purposes) when the comma is the last character of the line.