docs: correct MQTT example and ESP8266 guidance

README.md

@@ -8,7 +8,11 @@ ArduinoHA lets an Arduino, ESP8266, or ESP32 application publish MQTT discovery
 ## Start with a sensor
 
 ```cpp
+#if defined(ESP8266)
 #include <ESP8266WiFi.h>
+#elif defined(ESP32)
+#include <WiFi.h>
+#endif
 #include <ArduinoHA.h>
 
 WiFiClient client;
@@ -24,11 +28,11 @@ void setup() {
     // Your application connects Wi-Fi before MQTT begins.
     temperature.setName("Temperature");
     temperature.setUnitOfMeasurement("°C");
-    mqtt.begin("mqtt.local", "mqtt_user", "mqtt_password");
+    mqtt.begin("mqtt.local", "mqtt_user", "mqtt_password");  // Connection work begins in mqtt.loop().
 }
 
 void loop() {
-    mqtt.loop();
+    mqtt.loop();  // Maintains MQTT and publishes discovery after connecting.
     // Call temperature.setValue(...) when your reading changes.
 }
 ```