Add readme for nRF51 example

And pre-create directories for SDK for both nRF51 and nRF52.

PUBLISHED_FROM=00caf156f48da92804c6af8efe323535fbc5497d

examples/nRF51/README.md

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+# nRF51 example project
+
+This is a Mongoose "Hello, world" that can be compiled under nRF51 IoT SDK.
+
+To make it work, you should download [nRF51 IoT SDK](http://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF51-IoT-SDK)
+and unpack it in the current directory as `nrf51_iot_sdk`.
+
+The project structure replicates that of the examples provided with the SDK:
+Keil uVision project is located at `boards/pca10028/arm5_no_packs`, ARM-GCC
+Makefile is located in `boards/pca10028/armgcc`..

examples/nRF51/nrf51_iot_sdk/README.md

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+Please download the [nRF51 IoT SDK](http://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF51-IoT-SDK) and unpack it into this directory.

examples/nRF52/nrf5_iot_sdk/README.md

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+Please download the [nRF5 IoT SDK](http://developer.nordicsemi.com/nRF5_IoT_SDK/) and unpack it into this directory.