Use Yarn instead of npm install (#1072)

* Use Yarn instead of npm install

There's a problem with npm install and the option to package afterward. With yarn, you don't have this. I know it may be another required technology, but we need to change the doc if we don't fix the problem soon.

(see issue #1065)

* Update README.md

README.md

@@ -38,12 +38,13 @@ First, clone the repo via git:
 git clone --depth=1 https://github.com/chentsulin/electron-react-boilerplate.git your-project-name
 ```
 
-And then install dependencies.
-**ProTip**: Install with [yarn](https://github.com/yarnpkg/yarn) for faster and safer installation:
+And then install dependencies with yarn.
 
 ```bash
-$ cd your-project-name && npm install
+$ cd your-project-name
+$ yarn
 ```
+**Note**: If you can't use [yarn](https://github.com/yarnpkg/yarn) for some reason, try `npm install`.
 
 ## Run