Make it clear that Foundation has to be imported (apple#246)

It's not from the get-go clear for beginners that Foundation has to be imported as well. Otherwise, you would end up with a bunch of compilation errors.

Author: johannes-codes

Documentation/01 Getting Started.md

@@ -223,6 +223,9 @@ OPTIONS:
 As promised, here's the complete `count` command, for your experimentation:
 
 ```swift
+import ArgumentParser
+import Foundation
+
 struct Count: ParsableCommand {
     static let configuration = CommandConfiguration(abstract: "Word counter.")