Caption a11y docs (#928)

* Add a11y section for captions

* Update UNRELEASED.md

UNRELEASED.md

@@ -25,6 +25,7 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 ### Documentation
 
 - Added `Stack.Item` properties and description to [style guide](https://polaris.shopify.com)’s ([#772](https://github.com/Shopify/polaris-react/pull/772))
+- Added accessibility recommendations for the caption component ([#928](https://github.com/Shopify/polaris-react/pull/928/))
 
 ### Development workflow
 

src/components/Caption/README.md

@@ -89,3 +89,31 @@ Use to provide details in situations where content is compact and space is tight
 ![Default caption](/public_images/components/Caption/ios/default@2x.png)
 
 <!-- /content-for -->
+
+---
+
+## 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 -->
+
+Follow best practices for [data visualizations](/design/data-visualizations) to ensure that the purpose of captions is clear to all merchants, including those with issues related to seeing or understanding data and complex information.
+
+<!-- /content-for -->