Add starting instruction for yarn (#84)

Author: redidizzy

README.md

@@ -89,7 +89,7 @@ Differences between vue-analytics and the Nuxt.js implementation:
 
 1. Clone this repository
 2. Install dependencies using `yarn install` or `npm install`
-3. Start development server using `npm run dev`
+3. Start development server using `yarn dev` or `npm run dev`
 
 ## License