Add Game Screen info for implementation-notes.md and move face node f…

…eedback from model.md to implementation-notes.md. See #83.

doc/implementation-notes.md

@@ -1 +1,16 @@
-TODO implementation overview
+# Number Play - implementation notes
+
+## Game screen
+
+Both games rely on the abstract classes
+[NumberPlayGameLevel](https://github.com/phetsims/number-play/blob/master/js/game/model/NumberPlayGameLevel.ts) and
+[NumberPlayGameLevelNode](https://github.com/phetsims/number-play/blob/master/js/game/view/NumberPlayGameLevelNode.ts).
+
+The 'Counting' game builds upon the play areas that are used in the _Ten_, _Twenty_, and _Compare_ screens, but doesn't
+include the controls for changing the object type or adding/removing objects.
+[Subitizer](https://github.com/phetsims/number-play/blob/master/js/game/model/Subitizer.ts) controls the game sequence
+and determines the shapes to show in the 'Subitize' game.
+
+[NumberPlayGameAnswerButtons](https://github.com/phetsims/number-play/blob/master/js/game/view/NumberPlayGameAnswerButtons.ts)
+are used in both games for the user to input their answer. A smiley face is shown when the correct answer is selected. 
+When a wrong answer button is pressed, the button is disabled, and a frowny face is briefly shown. 

doc/model.md

@@ -55,7 +55,5 @@ Subitize | 2 | 6 | 10 |
 
 Since the challenges in both levels of each game are randomly generated, all levels can be played endlessly.
 One star is awarded per challenge that is successfully completed on the first try. On the 'Subitize' game,
-pressing the 'Reveal' button does not disqualify the user from getting a first-try star. Regardless of the 
-attempt number, the user always gets a smiley face when the correct answer is selected. When a wrong answer
-button is pressed, the button is disabled, and a frowny face is briefly shown. 
+pressing the 'Reveal' button does not disqualify the user from getting a first-try star.