Update README.md

Author: coryhouse

README.md

@@ -19,7 +19,8 @@ This will run the automated build process, start up a webserver, and open the ap
 3. Make sure files with names that begin with a dot (.babelrc, .editorconfig, .eslintrc) are copied to the project directory root. This is easy to overlook if you copy this repository manually.
 4. Don't run the project from a symbolic link. It will cause issues with file watches.
 5. Having linting issues? Delete any .eslintrc that you're storing in your user directory. Also, disable any ESLint plugin / custom rules that you've enabled within your editor. These will conflict with the ESLint rules defined in the course.
-6. Nothing above work? Delete your node_modules folder and re-run npm install.
+6. Seeing `Error: listen EADDRINUSE :::3000`? That means port 3000 is already in use on your machine. You probably have another instance of this project running on your machine in a different window. So find that window and kill the other instance using Ctrl+C.
+7. Nothing above work? Delete your node_modules folder and re-run npm install.
 
 ### Development Dependencies