Add readme

README.md

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+
+# Apache Mynewt NimBLE
+
+## Overview
+
+See (https://mynewt.apache.org/network/ble/ble_intro/).
+
+## Building
+
+NimBLE is usually built as a part of Apache Mynewt OS, but ports for
+other RTOS-es are also available.
+
+### Apache Mynewt
+
+(tbd)
+
+### FreeRTOS on nRF52 DK
+
+1. Download and install NRF5 SDK
+
+You will need to download nRF5 SDK from Nordic Semiconductor website:
+(https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF5-SDK).
+
+2. Make nRF5 SDK available for sample application
+
+Sample application expects nRF5 SDK to be available at location specified
+by `NRF5_SDK_ROOT` variable. By default it points to `./nrf5_sdk` directory
+(i.e. subdirectory of `porting/freertos_nrf5_sdk` directory) so the easiest
+way is to either copy nRF5 SDK to that location or make a symbolic link:
+
+```no-highlight
+    $ ln -s <x>/nRF5_SDK_12.3.0_d7731ad porting/freertos_nrf5_sdk/nrf5_sdk
+```
+
+Alternatively, you may want to overwrite default value of `NRF5_SDK_ROOT`
+to point to proper location:
+
+```no-highlight
+    $ export NRF5_SDK_ROOT=<x>/nRF5_SDK_12.3.0_d7731ad
+```
+
+3. Build and flash FreeRTOS image
+
+Provided Makefile is compatible with nRF5 SDK build system so sample
+application is built and flashed as other examples in nRF5 SDK:
+
+```no-highlight
+    $ make -C porting/freertos_nrf5_sdk
+    $ make -C porting/freertos_nrf5_sdk flash
+````
+