Use a problem as the intro for newtype pattern (#153)

A problem can highlight some of the use cases of newtype
pattern rather than discussing generally that rust is strongly
typed. Make the user understand the need of newtype.

Author: pickfire

patterns/newtype.md

@@ -1,8 +1,13 @@
 # Newtype
 
-Rust has strong static types. This can be very different than what you are used to if you are coming from a loosely-typed language. Don't worry, though. Once you get used to them, you'll find the types actually make your life easier. Why? Because you are making implicit assumptions explicit.
+What if in some cases we want a type to behave similar to another type or 
+enforce some behaviour at compile time where using only type aliases would 
+not be enough? 
 
-A really convenient application of the Rust type system is the Newtype pattern.
+For example, if we want to create a custom `Display` implementation for `String` 
+due to security considerations (e.g. passwords).
+
+For such cases we could use the `Newtype` pattern to provide __type safety__ and __encapsulation__.
 
 ## Description
 
@@ -97,7 +102,7 @@ Here, `Bar` might be some public, generic type and `T1` and `T2` are some intern
 
 ## See also
 
-- [Newtypes in the style guide](https://doc.rust-lang.org/1.0.0/style/features/types/newtype.html)
+- [Advanced Types in the book](https://doc.rust-lang.org/book/ch19-04-advanced-types.html?highlight=newtype#using-the-newtype-pattern-for-type-safety-and-abstraction)
 - [Newtypes in Haskell](https://wiki.haskell.org/Newtype)
 - [Type aliases](https://doc.rust-lang.org/stable/book/ch19-04-advanced-types.html#creating-type-synonyms-with-type-aliases)
 - [derive_more](https://crates.io/crates/derive_more), a crate for deriving many builtin traits on newtypes.