chore: Add a readme for application developers (#506)

Author: drewcrawford

README-APPLICATION-DEVELOPERS.md

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+# Tips for application developers
+
+## Introduction
+
+Please note: This document is a work in progress and will be expanded over time to include details
+about how to make accessible applications using AccessKit.
+
+## Other Resources
+
+Many accessible technologies have their own tips for application developers.  See:
+
+* [Orca's Tips for Application Developers](https://gitlab.gnome.org/GNOME/orca/-/blob/main/README-APPLICATION-DEVELOPERS.md?ref_type=heads&plain=0)
+* [Apple's accesibility HIG](https://developer.apple.com/design/human-interface-guidelines/accessibility)
+
+## Using screen readers
+
+The best way to test applications for accessibility is with a screen reader.  Here are some key commands to know:
+
+### macOS
+
+* Turn VoiceOver on or off with `Command + F5`  (Some Macs require `Command + Function + F5`.  If your Mac or Magic Keyboard has Touch ID, press and hold the Command key while you quickly press Touch ID three times.)
+* Move the VoiceOver cursor with `Control + Option + Arrow keys`, or by clicking with the mouse.
+* Perform the default action with `Control + Option + Space`.
+* Access a menu with `Control + Option + H + H` (pressing the H key twice).  This provides a menu of all commands including developer ones.
+* More commands [here](https://support.apple.com/guide/voiceover/general-commands-cpvokys01/mac).
+
+### Linux
+
+orca can be enabled from the settings screen in many desktop environments, as well as by running `orca` on the command line.  `orca --setup` will
+open a configuration pane, and `orca --debug-file=file.log` will log debug information to a file which can be useful for
+debugging applications.
+
+orca's keyboard configurations come in two flavors, the 'Desktop' (default) and 'Laptop' configurations. This document discusses
+the Desktop flavor.
+
+* 'Where am I?' is `Keypad Enter` (press twice for more info)
+* Move focus with `Insert + Ctrl + Arrow key`
+* Speak current item with `Insert + Shift + Up Arrow`
+* Present object actions with `Insert-Shift-a`
+* Speak entire window with Flat Review with `Keypad Add + Keypad Add` (pressing the plus key twice)
+* To get a menu of all shortcuts on your system, use `Insert + H` to enter learn mode, and then `F2`.
+* More information about Orca is available [here](https://help.gnome.org/users/orca/stable/index.html.en).
+
+Problems with orca can sometimes be resolved by removing user settings at `~/.local/share/orca` and relaunching orca.
+
+
+## Using accessibility inspectors
+
+Accessibility inspectors provide a convenient way to inspect your accessibility tree.  However, they can be very
+misleading in terms of how screen reader users experience an application.
+
+### macOS
+
+on macOS you can use the Accessibility Inspector, which is part of Xcode.  See the documentation at [Apple's developer site](https://developer.apple.com/documentation/accessibility/accessibility-inspector).
+
+Accessibilty Inspector provides GUI buttons for navigation through the application and visually displays properties.  It includes
+some auditing tools to look for common issues in your application.
+
+The macOS tooling is of reasonably high quality for common tasks but does not always surface issues that are important
+for screen reader users.
+
+### Linux
+
+The GNOME project provides the [accerciser](https://help.gnome.org/users/accerciser/stable/introduction.html.en) inspector.
+It provides a basic tree view of the accessibility tree and can be used to inspect properties of nodes and run
+some simple audits.
+
+accerciser has many limitations.  It often fails to show node properties in complex trees and cannot display the tree
+for the orca setup window for example.  It often hangs and crashes on complex trees, it does not provide a facility to focus or navigate nodes in the tree or send
+most events to applications. And it will not display
+trees created by accesskit on its own because it does not enable the ATSPI bus.  (To do this, turn on a screenreader or
+run this command: `busctl --user set-property org.a11y.Bus /org/a11y/bus org.a11y.Status IsEnabled b true`
+
+The KDE project provides the [Accessibility Inspector](https://apps.kde.org/accessibilityinspector/) application.  In general
+it is superior to accerciser, it can display the tree for orca's setup window, it provides a checkbox to enable the ATSPI bus,
+and it supports focusing and navigating nodes in the tree to some extent.  However it still frequently crashes and hangs
+and provides no audit functionality.
+
+Both inspectors often display information that is totally inaccessible to screen reader users and so relying on them
+to test accessibility can be misleading.
+
+## About virtualization
+
+Many developers use virtualization to test their software on other platforms.  However, using virtualization to test accessibility
+has some unique challenges.
+
+### Sound
+
+Linux often has choppy audio under virtualization due to its small audio buffers, and this may make it difficult to hear if
+a screen reader is reading or to understand its speech.  Placing the following in
+`~/.config/wireplumber/wireplumber.conf.d/50-alsa-config.conf` can help:
+
+```
+monitor.alsa.rules = [
+  {
+    matches = [
+      # This matches the value of the 'node.name' property of the node.
+      {
+        node.name = "~alsa_output.*"
+      }
+    ]
+    actions = {
+      # Apply all the desired node specific settings here.
+      update-props = {
+        api.alsa.period-size   = 1024
+        api.alsa.headroom      = 8192
+      }
+    }
+  }
+]
+```
+
+### Input
+
+Popular virtualization software interferes with the keyboard shortcuts used with screen readers.  This can cause
+screen readers to seemingly not respond, or respond incorrectly or inconsistently to keyboard commands.
+
+One workaround is to mount a keyboard for the exclusive use of the guest.  (A full-size keyboard is recommended since
+screen readers use many keys.)  This feature is disabled by default in VMWare; to enable it, append the following
+to the vmx file:
+
+```angular2html
+usb.generic.allowHID = "TRUE"
+usb.generic.allowLastHID = "TRUE"
+```