Update tuple-structs.md (#226)

* Update tuple-structs.md

I am adding more information about newtypes. The speaker can get questions about it or may wish to encourage this great pattern.

* Fix typos and explanation

Co-authored-by: Andrew Walbran <qwandor@google.com>

Author: igor-petruk

src/structs/tuple-structs.md

@@ -31,3 +31,11 @@ fn main() {
 }
 
 ```
+
+<details>
+
+Newtypes are a great way to encode additional information about the value in a primitive type, for example:
+  * The number is measured in some units: `Newtons` in the example above.
+  * The value passed some validation when it was created, so you no longer have to validate it again at every use: 'PhoneNumber(String)` or `OddNumber(u32)`.
+    
+</details>