Add i18n docs and update README

docs/guides/internationalization.md

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+# Internationalization
+
+To support right-to-left languages, application layout can be automatically
+flipped from LTR to RTL. The `I18nManager` API can be used to help with more
+fine-grained control and testing of RTL layouts.
+
+React Native for Web provides an experimental feature to support "true left"
+and "true right" styles. For example, `left` will be flipped to `right` in RTL
+mode, but `left$noI18n` will not. More information is available in the `Text`
+and `View` documentation.
+
+## Working with icons and images
+
+Icons and images that must match the LTR or RTL layout of the app need to be manually flipped.
+
+Either use a transform style:
+
+```js
+<Image
+  source={...}
+  style={{ transform: [{ scaleX: I18nManager.isRTL ? -1 : 1 }] }}
+/>
+```
+
+Or replace the source asset:
+
+```js
+import imageSourceLTR from './back.png';
+import imageSourceRTL from './forward.png';
+
+<Image
+  source={I18nManager.isRTL ? imageSourceRTL : imageSourceLTR}
+/>
+```