Accessibility: Added VoiceOver (macOS) and Narrator (Windows) accessibility screen reader support to juce_gui_basics

docs/Accessibility.md

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+# JUCE Accessibility
+
+## What is supported?
+
+Currently JUCE supports VoiceOver on macOS and Narrator on Windows. The JUCE
+accessibility API exposes the following to these clients:
+
+  - Title, description, and help text for UI elements
+  - Programmatic access to UI elements and text
+  - Interaction with UI elements
+  - Full UI keyboard navigation
+  - Posting notifications to listening clients
+
+## Customising Behaviour
+
+By default any visible and enabled `Component` is accessible to screen reader
+clients and exposes some basic information such as title, description, help
+text and its position in the hierarchy of UI elements.
+
+The `setTitle()`, `setDescription()` and `setHelpText()` methods can be used
+to customise the text that will be read out by accessibility clients when
+interacting with UI elements and the `setExplicitFocusOrder()`,
+`setFocusContainer()` and `createFocusTraverser()` methods can be used to
+control the parent/child relationships and the order of navigation between UI
+elements.
+
+## Custom Components
+
+For further customisation of accessibility behaviours the `AccessibilityHandler`
+class provides a unified API to the underlying native accessibility libraries.
+
+This class wraps a component with a given role specified by the
+`AccessibilityRole` enum and takes a list of optional actions and interfaces to
+provide programmatic access and control over the UI element. Its state is used
+to convey further information to accessibility clients via the
+`getCurrentState()` method.
+
+To implement the desired behaviours for a custom component, subclass
+`AccessibilityHandler` and return an instance of this from the
+`Component::createAccessibilityHandler()` method.
+
+Examples of some common UI element handlers for existing JUCE widgets can be
+found in the [`widget_handlers`](/modules/juce_gui_basics/accessibility/widget_handlers) directory.
+
+## Further Reading
+
+  - [NSAccessibility protocol](https://developer.apple.com/documentation/appkit/nsaccessibility?language=objc)
+  - [UI Automation for Win32 applications](https://docs.microsoft.com/en-us/windows/win32/winauto/entry-uiauto-win32)
+  - A talk giving an overview of this feature from ADC 2020 can be found on
+    YouTube at https://youtu.be/BqrEv4ApH3U
+