added example how to get rtc counters

Author: Heltec-Aaron-Lee

examples/ESP32/RTC_counter/RTC_counter.ino

@@ -0,0 +1,18 @@
+#include "Arduino.h"
+#include "soc/rtc.h"
+
+/*rtc_time_get() return the RTC counters
+* the board using internal 150KHz RTC, but it's not accurate.
+*/
+void setup() {
+  // put your setup code here, to run once:
+  Serial.begin(115200);
+}
+
+void loop() {
+  uint64_t rtc_counter1 = rtc_time_get();
+  delay(1000);
+  uint64_t rtc_counter2 = rtc_time_get();
+  
+  Serial.println((uint32_t)(rtc_counter2-rtc_counter1));
+}