[a11y] Add empty state component a11y docs (#2244)

* add a11y docs

* add changes to UNRELEASED.md

UNRELEASED.md

@@ -18,6 +18,8 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 
 ### Documentation
 
+- Added accessibility documentation for the empty state component ([#2244](https://github.com/Shopify/polaris-react/pull/2244))
+
 ### Development workflow
 
 ### Dependency upgrades

src/components/EmptyState/README.md

@@ -204,3 +204,31 @@ Use to provide additional but non-critical context for a new product or feature.
 - To learn more about illustrations for empty states, [read the illustration guidelines](https://polaris.shopify.com/design/illustrations)
 - To create page-level layout, [use the layout component](https://polaris.shopify.com/components/structure/layout)
 - To highlight a Shopify feature, [use the callout card component](https://polaris.shopify.com/components/structure/callout-card)
+
+---
+
+## Accessibility
+
+<!-- content-for: android -->
+
+See Material Design and development documentation about accessibility for Android:
+
+- [Accessible design on Android](https://material.io/design/usability/accessibility.html)
+- [Accessible development on Android](https://developer.android.com/guide/topics/ui/accessibility/)
+
+<!-- /content-for -->
+
+<!-- content-for: ios -->
+
+See Apple’s Human Interface Guidelines and API documentation about accessibility for iOS:
+
+- [Accessible design on iOS](https://developer.apple.com/design/human-interface-guidelines/ios/app-architecture/accessibility/)
+- [Accessible development on iOS](https://developer.apple.com/accessibility/ios/)
+
+<!-- /content-for -->
+
+<!-- content-for: web -->
+
+Empty state illustrations are implemented as decorative images, so they use an empty `alt` attribute and are skipped by technologies like screen readers.
+
+<!-- /content-for -->