Update README.md for first use case

an extra step for the first out of box experience required for nrf52, took me a few hours to test why the following did not work:
```
void setup() {
  pinMode(PIN_LED1, OUTPUT);
  pinMode(PIN_LED2, OUTPUT);
  pinMode(PIN_LED3, OUTPUT);
  pinMode(PIN_LED4, OUTPUT);
}

void loop() {
  digitalWrite(PIN_LED1, HIGH);
  delay(50);
  digitalWrite(PIN_LED2, HIGH);
  delay(50);
  digitalWrite(PIN_LED3, HIGH);
  delay(50);
  digitalWrite(PIN_LED4, HIGH);
  delay(50);
  digitalWrite(PIN_LED1, LOW);
  delay(50);
  digitalWrite(PIN_LED2, LOW);
  delay(50);
  digitalWrite(PIN_LED3, LOW);
  delay(50);
  digitalWrite(PIN_LED4, LOW);
  delay(50);
}
```

Author: nihaopaul

README.md

@@ -5,6 +5,7 @@ Program [Nordic Semiconductor](https://www.nordicsemi.com) nR5 based boards with
 ## Supported boards
 
  * [nRF52 DK](https://www.nordicsemi.com/eng/Products/Bluetooth-Smart-Bluetooth-low-energy/nRF52-DK)
+   * First time use - To get started, you need to power on while depressing boot/reset button and copy over the latest [bootloader](https://www.nordicsemi.com/eng/nordic/Products/nRF52-Preview-DK/nRF5x-DK-J-Link-IF/49235) before it responds to sketches uploaded.
  * [Bluz DK](http://bluz.io)
 
 ## Installing