Add yarn example under "Installing a Dependency" (#2362)

Due to many of the other examples in the README showing both `npm` and `yarn` commands. I have added an example of how to install a dependency using `yarn`.
wmonk · Aug 7, 2017 · 2bd9c8b · 2bd9c8b
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diff --git a/packages/react-scripts/template/README.md b/packages/react-scripts/template/README.md
@@ -281,6 +281,12 @@ The generated project includes React and ReactDOM as dependencies. It also inclu
 npm install --save <library-name>
 ```
 
+Alternatively you may also use `yarn`:
+
+```
+yarn add <library-name>
+```
+
 ## Importing a Component
 
 This project setup supports ES6 modules thanks to Babel.<br>