Document how to get the playground to work

Refs #644

PromiseKit.playground/contents.xcplayground

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-<playground version='3.0' sdk='iphonesimulator' auto-termination-delay='2'>
+<playground version='3.0' sdk='macosx' auto-termination-delay='2'>
     <sections>
         <code source-file-name='section-1.swift'/>
     </sections>

PromiseKit.playground/section-1.swift

@@ -1,14 +1,29 @@
 import PlaygroundSupport
+
+// Is this erroring? If so open the `.xcodeproj` and build the
+// framework for a macOS target (usually labeled: “My Mac”).
+// Then select `PromiseKit.playground` from inside Xcode.
 import PromiseKit
 
+
+func promise3() -> Promise<Int> {
+    return after(interval: 1).then {
+        return 3
+    }
+}
+
 firstly {
     Promise(value: 1)
 }.then { _ in
     2
 }.then { _ in
-    3
+    promise3()
+}.then {
+    print($0)  // => 3
+}.always {
+    // always happens
 }.catch { error in
-    // never happens!
+    // only happens for errors
 }
 
 PlaygroundPage.current.needsIndefiniteExecution = true