Add .env.example file for environment variable configuration

- Created a new .env.example file to provide a template for required environment variables, including NODE_ENV, HASS_HOST, and HASS_TOKEN.
- This addition enhances configuration management by guiding users on necessary settings for the application.

.env.example

@@ -0,0 +1,3 @@
+NODE_ENV=production
+HASS_HOST=your_home_assistant_url
+HASS_TOKEN=your_home_assistant_token