Add documentation for Show

docs/src/main/tut/typeclasses/show.md

@@ -7,3 +7,68 @@ scaladoc: "#cats.Show"
 ---
 # Show
 
+Show is an alternative to the Java `toString` method.
+It is defined by a single function `show`:
+
+```scala
+def show(a: A): String
+```
+
+You might be wondering why you would want to use this, considering `toString` already serves the same purpose and case classes already provide sensible implementations for `toString`.
+The difference, is that `toString` is defined on `Any`(Java's `Object`) and can therefore be called on anything, not just case classes.
+Most often, this is unwanted behaviour, as the standard implementation of `toString` on non case classes is mostly gibberish.
+Consider the following example:
+
+```tut:book
+(new {}).toString
+```
+
+The fact that this code compiles is a design flaw of the Java API.
+We want to make things like this impossible, by offering the `toString` equivalent as a type class, instead of the root of the class hierarchy.
+In short, `Show` allows us to only have String-conversions defined for the data types we actually want.
+
+To make things easier, cats defines a few helper functions to make creating `Show` instances easier.
+
+```scala
+/** creates an instance of Show using the provided function */
+def show[A](f: A => String): Show[A]
+
+/** creates an instance of Show using object toString */
+def fromToString[A]: Show[A]
+```
+
+These can be used like this:
+
+```tut:book
+import cats.Show
+
+case class Person(name: String, age: Int)
+
+implicit val showPerson: Show[Person] = Show.show(person => person.name)
+
+case class Department(id: Int, name: String)
+
+implicit val showDep: Show[Department] = Show.fromToString
+```
+
+
+This still may not seem useful to you, because case classes already automatically implement `toString`, while `show` would have to be implemented manually for each case class.
+Thankfully with the help of a small library called [kittens](https://github.com/milessabin/kittens)a lot of type class instances including `Show` can be derived automatically!
+
+Cats also offers `Show` syntax to make working with it easier.
+This includes the `show` method which can be called on anything with a `Show` instance in scope:
+
+```tut:book
+import cats.implicits._
+
+val john = Person("John", 31)
+
+john.show
+```
+
+It also includes a String interpolator, which works just like the standard `s"..."` interpolator, but uses `Show` instead of `toString`:
+
+```tut:book
+val engineering = Department(2, "Engineering")
+show"$john works at $engineering"
+```