switch to yarn (#2)

README.md

@@ -10,20 +10,20 @@ This is the web frontend for Vikunja, written in Vue.js.
 
 ## Project setup
 ```
-npm install
+yarn install
 ```
 
 ### Compiles and hot-reloads for development
 ```
-npm run serve
+yarn run serve
 ```
 
 ### Compiles and minifies for production
 ```
-npm run build
+yarn run build
 ```
 
 ### Lints and fixes files
 ```
-npm run lint
+yarn run lint
 ```