README.md: add instructions for softdevice (sandeepmistry#129)

* README.md: add instructions for softdevice

* README.md: add instructions for softdevice

* Update README.md

Author: jacobrosenthal

README.md

@@ -102,6 +102,12 @@ cd ~/Documents/mpide/libraries/
 git clone https://github.com/sandeepmistry/arduino-BLEPeripheral BLEPeripheral
 ```
 
+### [arduino-nRF5x core](https://github.com/sandeepmistry/arduino-nRF5) users
+
+The [arduino-nRF5x core](https://github.com/sandeepmistry/arduino-nRF5) **REQUIRES** a SoftDevice in order to successfully use this library. Please see [Flashing a SoftDevice](https://github.com/sandeepmistry/arduino-nRF5#flashing-a-softdevice).
+
+Further, the error ```fatal error: ble_gatts.h: No such file or directory``` in indicitive of no SoftDevice being selected.
+
 ### Starter sketch
 Load [starter.ino](examples/starter/starter.ino)