add InterruptIn examples

APIs_Drivers/InterruptIn_ex_1/README.md

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+# InterruptIn example
+
+This example shows how to use InterruptIn to trigger a function call when a certain event happens. The example demonstrates calling a function on the rising edge of a button press. You can observe this through the counter value changing when the button is pressed.

APIs_Drivers/InterruptIn_ex_1/main.cpp

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+/*
+ * Copyright (c) 2017-2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "mbed.h"
+
+class Counter {
+public:
+    Counter(PinName pin) : _interrupt(pin)          // create the InterruptIn on the pin specified to Counter
+    {
+        _interrupt.rise(callback(this, &Counter::increment)); // attach increment function of this counter instance
+    }
+
+    void increment()
+    {
+        _count++;
+    }
+
+    int read()
+    {
+        return _count;
+    }
+
+private:
+    InterruptIn _interrupt;
+    volatile int _count;
+};
+
+Counter counter(SW2);
+
+int main()
+{
+    while (1) {
+        printf("Count so far: %d\n", counter.read());
+        ThisThread::sleep_for(2000);
+    }
+}

APIs_Drivers/InterruptIn_ex_2/README.md

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+# InterruptIn example
+
+This example shows how to use InterruptIn to trigger a function call when a certain event happens. The example demonstrates calling a function on the rising edge of a button press. You can observe this through LED4 blinking as the program runs. LED1 only changes when the button is pressed. 

APIs_Drivers/InterruptIn_ex_2/main.cpp

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+/*
+ * Copyright (c) 2017-2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "mbed.h"
+
+InterruptIn button(SW2);
+DigitalOut led(LED1);
+DigitalOut flash(LED4);
+
+void flip()
+{
+    led = !led;
+}
+
+int main()
+{
+    button.rise(&flip);  // attach the address of the flip function to the rising edge
+    while (1) {          // wait around, interrupts will interrupt this!
+        flash = !flash;
+        ThisThread::sleep_for(250);
+    }
+}