Add iOS VoiceOver doc.

docs/ios_voiceover.md

+# VoiceOver
+
+VoiceOver is an Accessibility feature available on Apple platforms. It provides
+an interface between your app and your blind or low-vision users. In short, it's
+a screen reader.
+
+## Overview
+
+[Activating VoiceOver](#Activate-VoiceOver) fundamentally changes the touch
+interface of an iOS device, so when you activate it, you need to know these
+basic commands to navigate:
+
+-   VoiceOver focuses an item on the screen and speaks it to the user.
+-   To navigate between items, you can either flick left/right to go to the
+    previous/next item, or you can drag your finger around the screen.
+-   Double tapping anywhere on the screen activates the item (no need to double
+    tap exactly on the item on the screen).
+-   Items have a label (describing the item) and can have among others:
+    -   traits (describing the item: a header, a button, selection state, etc.);
+    -   a hint (describing how to interact with the item).
+
+The following video is a great summary. The content is succinct, high quality
+and up-to-date, even though the video quality is low and the demo is on an
+iPhone 3GS.
+
+[![Video screenshot]][Video link]
+
+## Activate VoiceOver
+
+Go to `Settings > General > Accessibility > VoiceOver` and toggle the top
+switch.
+
+Alternatively, if you are activating/deactivating VoiceOver often, I'd advise
+to set it as the Accessibility Shortcut. When enabled, a triple tap on the Home
+button will toggle VoiceOver ON and OFF from anywhere.
+```
+Settings > General > Accessibility > Accessibility Shortcut
+```
+
+## Additional Resources
+
+-   [Apple presentation of VoiceOver]
+-   [AppleVis intro to VoiceOver]
+
+[Apple presentation of VoiceOver]: http://www.apple.com/accessibility/osx/voiceover/
+[AppleVis intro to VoiceOver]: http://www.applevis.com/guides/ios-miscellaneous-voiceover/intro-ios-accessibility-blind-and-low-vision-users
+[Video screenshot]: https://img.youtube.com/vi/WxQ2qKShvmc/0.jpg
+[Video link]: https://www.youtube.com/watch?v=WxQ2qKShvmc "iPhone VoiceOver Function For People With Disabilities"