add an information in contributing doc to have the latest version of yarn (codesandbox#2662)

Author: mkamranhamid

CONTRIBUTING.md

@@ -64,6 +64,7 @@ To install the project you need to have `yarn` and `node`
     - you can use [fnm](https://github.com/Schniz/fnm) (`fnm use`) to change
       your current node version to the one specified in `.nvmrc`
 3.  from the root of the project: `yarn` to install all dependencies
+    - make sure you have latest `yarn` version 
 4.  from the root of the project: `yarn start`
     - this builds the dependencies (`codesandbox-api` and
       `codesandbox-browserfs`) and runs the `app` development environment,