Add ESP32 examples folder and sigma-delta example

libraries/ESP32/examples/AnalogOut/SigmaDelta/SigmaDelta.ino

+void setup()
+{
+    //setup channel 0 with frequency 312500 Hz
+    sigmaDeltaSetup(0, 312500);
+    //attach pin 18 to channel 0
+    sigmaDeltaAttachPin(18,0);
+    //initialize channel 0 to off
+    sigmaDeltaWrite(0, 0);
+}
+
+void loop()
+{
+    //slowly ramp-up the value
+    //will overflow at 256
+    static uint8_t i = 0;
+    sigmaDeltaWrite(0, i++);
+    delay(100);
+}

libraries/ESP32/library.properties

+name=ESP32
+version=1.0
+author=Hristo Gochkov, Ivan Grokhtkov
+maintainer=Hristo Gochkov <hristo@espressif.com>
+sentence=ESP32 sketches examples
+paragraph=
+category=Other
+url=
+architectures=esp32

libraries/ESP32/src/dummy.h

+// This file is here only to silence warnings from Arduino IDE
+// Currently IDE doesn't support no-code libraries, like this collection of example sketches.