Add compile-tested support for ESP32-IDF, some fixes from code review,

update README

components/dmx512/dmx512esp32idf.h

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+#pragma once
+
+#ifdef USE_ESP_IDF
+
+#include "dmx512.h"
+
+namespace esphome {
+namespace dmx512 {
+
+class DMX512ESP32IDF : public DMX512 {
+ public:
+  DMX512ESP32IDF() = default;
+
+  void sendBreak() override;
+
+  void set_uart_num(int num) override {
+    this->uart_idx_ = num;
+  }
+};
+
+}  // namespace dmx512
+}  // namespace esphome
+
+#endif  // USE_ESP_IDF