[docs] Suggest to install lib as prod dependency (#689)

Code of package runs in production,
thus we should install it with --save, not with --save-dev

fixes #681

Author: Dima Krutolianov

README.md

@@ -95,7 +95,7 @@ const store = createStore(reducer, enhancer);
 
 To make things easier, there's an npm package to install:
 ```
-npm install --save-dev redux-devtools-extension
+npm install --save redux-devtools-extension
 ```
 and to use like so:
 ```js