Update README.md

Author: guojiex

README.md

@@ -3,6 +3,8 @@
 A sample [Google Custom Search](https://developers.google.com/custom-search/) Flutter demo.
 It uses [Custom Search JSON API](https://developers.google.com/custom-search/v1/overview) to get search result.
 
+We use [expire_cache](https://github.com/guojiex/expire_cache) to make sure the same search request will not be fired multiple times(with TTL for the cache entry).
+
 ## Getting Started
 
 This project is a starting point for a Flutter application.
@@ -24,4 +26,4 @@ samples, guidance on mobile development, and a full API reference.
 
 ### new CSE websearch layout
 
-![cse_websearch_layout_gif](/res/imgs/examples/20190116.gif)
+![cse_websearch_layout_gif](/res/imgs/examples/20190116.gif)