Add example of getting window dimensions to Dimensions docs

By @calvinf

Closes https://github.com/facebook/react-native/pull/4211

Libraries/Utilities/Dimensions.js

@@ -57,6 +57,8 @@ class Dimensions {
    * than caching the value (for example, using inline styles rather than
    * setting a value in a `StyleSheet`).
    *
+   * Example: `var {height, width} = Dimensions.get('window');`
+   *
    * @param {string} dim Name of dimension as defined when calling `set`.
    * @returns {Object?} Value for the dimension.
    */