tour: Anonymous Struct Not Explained

[mattc41190]

This issue is a lot like #412. In https://tour.golang.org/moretypes/9 there is usage of an anonymous struct. This is the first time a new comer is introduced to the concept that a composite type may be informally declared.

As a suggestion I would add this after https://tour.golang.org/moretypes/5 with the following simple explanation:

"Structs may be declared inline and without a name if the need arises for a one time use structure."

package main

import "fmt"

func main() {
	fmt.Println(struct{Greeting, Planet string}{"Hello", "World"})
}

The pros to including this explanation is that users are not caught off guard when they find it elsewhere in the tour or in the wild. The cons are that it may encourage less than perfectly idiomatic go code, but if that were the case the allowance of their existence in the spec is suspect.

Happy to create this if you guys would like :)