add yarn to readme

Author: jscottsmith

README.md

@@ -15,10 +15,18 @@ Some links demonstrating possible effects created with this lib:
 
 ## Install
 
+With npm
+
 ```
 npm i react-scroll-parallax --save
 ```
 
+or yarn
+
+```
+yarn add react-scroll-parallax
+```
+
 ## Usage
 
 The `<ParallaxProvider />` should wrap the component tree that contains all `<Parallax />` components. This should be a top level component like `<AppContainer />`. The `<ParallaxProvider />` will then provide necessary context to the [`parallaxController`](#parallax-controller-context) for all `<Parallax />` elements. For example: