Add docs for running the project locally

README.md

@@ -1,10 +1,33 @@
 # Endless City 🏢
 WebGL city scene heavily inspired by https://demos.littleworkshop.fr/infinitown
-## How to run
-Just open https://corashina.github.io/Endless-City/
+
+## Demo
+https://corashina.github.io/Endless-City
+
+## Running the project locally
+If you just open the `index.html` file in a browser, you will get a CORS error that says something like this:
+
+```
+Access to XMLHttpRequest at 'file:///path-to-project/Endless-City/js/clusters/road.gltf'
+from origin 'null' has been blocked by CORS policy: Cross origin requests are only
+supported for protocol schemes: http, data, chrome, chrome-extension, https.
+```
+
+This is because browsers give different privileges and treatment to files if they’re local or on a web server.
+
+To run this file without that error, you can test a local file out as if it were on a web server by following these steps:
+
+```sh
+cd Endless-City
+python3 -m http.server 1234
+```
+
+Then navigate to http://localhost:1234 and voila!
+
 ## Features
  - Map camera controls with damping
  - Infinite map
  - Cars
+
 ## Preview
 ![alt text](https://raw.githubusercontent.com/Tomasz-Zielinski/Endless-City/master/preview.png)