error out when user tries to use NPM to install dependencies

CONTRIBUTING.md

@@ -11,7 +11,6 @@ xcode-select --install
 
 Install [node.js](https://nodejs.org/) version 4 or greater
 ```bash
-brew update
 brew install node
 ```
 Install [yarn](https://yarnpkg.com/en/)

package.json

@@ -126,6 +126,7 @@
     "test-flow": "flow .",
     "test-cov": "nyc --require=flow-remove-types/register --reporter=text-summary --reporter=lcov --cache run-s test-unit test-render test-query",
     "prepublish": "in-publish && run-s build-dev build-min || not-in-publish",
+    "preinstall": "node -e 'if(!/yarn\\.js$/.test(process.env.npm_execpath))throw new Error(\"NPM not supported, please use Yarn to install dependencies ($ yarn install).\")'",
     "postinstall": "cd src/style-spec && yarn || npm install"
   }
 }