[update] document the environment variables

README.md

@@ -29,3 +29,15 @@ npm run start:prod
 ```
 
 This will automatically run `npm run build` to generate the production ready files.
+
+## Enviroment variables
+- `NODE_ENV`: the envirment (default: `development`)
+- `PORT`: the private port (default: `3000`)
+- `HOST`: the private host (default: `localhost`)
+- `P_PROT`: the public protocol (default: `http`)
+- `P_PORT`: the public port (default: `3000`)
+- `P_HOST`: the public host (default: `localhost`)
+
+The private **port** and **host** are used to run the app inside a docker container.
+
+The public **port**, **host** and **protocol** are used to fetch data from the server in the React application.