update readme

packages/superset-ui-translation/README.md

@@ -9,13 +9,14 @@
 #### Example usage
 
 ```js
-import { configure, t } from '@superset-ui/translation';
+import { configure, t, tn } from '@superset-ui/translation';
 
 configure({
   languagePack: {...},
 });
 
 console.log(t('text to be translated'));
+console.log(tn('singular text', 'plural text', value));
 ```
 
 #### API
@@ -27,17 +28,28 @@ console.log(t('text to be translated'));
 
 `t(text[, args])`
 
-- Translate `text` when no `args` is provided.
-- Translate `text` and substitute `args` into the placeholders specified within `text`.
+- Translate `text`.
+- If `args` is provided, substitute `args` into the `sprintf` placeholders specified within `text` translation.
 
 For example
 
 ```js
-t('Hello %(name)s', user)
+t('Hello %(name)s', user);
 ```
 
 See [sprintf-js](https://github.com/alexei/sprintf.js) for more details on how to define placeholders.
 
+`tn(singular, plural, num, [, args])`
+
+- Translate and choose between `singular` and `plural` based on `num`.
+- If `args` is provided, substitute `args` into the `sprintf` placeholders specified within `singular` or `plural` translations.
+
+For example
+
+```js
+tn('%d duck', '%d ducks', 2, 2);
+```
+
 ### Development
 
 `@data-ui/build-config` is used to manage the build configuration for this package including babel