Add README.md for lib/translation (#10005)

src/lib/translation/README.md

@@ -0,0 +1,59 @@
+# Translation
+
+A hook for translating text on-device for inline display.
+
+## Translating text
+
+```tsx
+const langPrefs = useLanguagePrefs()
+const {translate} = useTranslate({key: post.uri})
+
+// ...
+
+void translate({
+  text: record.text,
+  targetLangCode: langPrefs.primaryLanguage,
+})
+```
+
+## Clearing/hiding a translation
+
+```tsx
+const {clearTranslation} = useTranslate({key: post.uri})
+
+// ...
+
+clearTranslation()
+```
+
+## Rendering a translation
+
+```tsx
+const {translationState} = useTranslate({key: post.uri})
+
+// ...
+
+switch (translationState.status) {
+  case 'idle':
+    // Default state; render a link that calls `translate`.
+    break;
+  case 'loading':
+    // On-device translation is in progress; render a loading spinner.
+    break;
+  case 'success':
+    // Translation complete; render `translationState.translatedText` and a link
+    // that calls `clearTranslation`.
+    break;
+  case 'error':
+    // On-device translation failed; render `translationState.message` and a
+    // link to `translate` from `useGoogleTranslate` as a fallback.
+    break;
+}
+```
+
+## Notes
+
+* Android only supports two-letter language codes.
+   * For example, this means it doesn’t differentiate between `pt-BR` and `pt-PT`.
+* Android and iOS only support a subset of the language options we offer (iOS supports fewer than Android).
+* Individual language packs must be downloaded on iOS.