Add Specify Port Of CRA's Webpack Dev Server as a webpack til

webpack/specify-port-of-cra-webpack-dev-server.md

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+# Specify Port Of CRA's Webpack Dev Server
+
+[`create-react-app`](https://github.com/facebookincubator/create-react-app)
+gives you a set of scripts, one of which allows you to start a development
+server that bundles and serves your javascript. This is handled under the
+hood via `webpack-dev-server`. By default it attempts to serve from port
+`3000`. If port `3000` is taken it will attempt to connect to another
+sequential port.
+
+Alternatively, you can just specify the port when starting the development
+server. This can be done with the `PORT` env var.
+
+```bash
+$ PORT=3333 yarn start
+```