Add example in docs for making arrays without keys (#506)

This was only discussed in issue #104 but not mentioned anywhere else.

Co-authored-by: David Heinemeier Hansson <[email protected]>

Author: ADTC

README.md

@@ -108,6 +108,15 @@ json.array! @people, :id, :name
 # => [ { "id": 1, "name": "David" }, { "id": 2, "name": "Jamie" } ]
 ```
 
+To make a plain array without keys, construct and pass in a standard Ruby array.
+
+``` ruby
+my_array = %w(David Jamie)
+
+json.people my_array
+
+# => "people": [ "David", "Jamie" ]
+
 You don't always have or need a collection when building an array.
 
 ```ruby