Add yarn (#830)

* estou mexendo ainda

* Update README.md

README.md

@@ -17,6 +17,13 @@ Bulma is in early but active development! Try it out now:
 ```sh
 npm install bulma
 ```
+**or**
+
+### YARN
+
+```sh
+yarn add bulma
+```
 
 ### Bower