doc: Add how to generate libnnstreamer.so with ndk-build

doc: Add how to generate libnnstreamer.so with ndk-build

This commit is to append a document to describe how to compile
the nnstreamer repository with the ndk-build command to get the
libnnstreamer for Android software platform. The ndk-build
is provided by Android NDK toolkit.

**Changes

* Version 2:
1. Added how to download (custom) prebuilt android-gstreamer files
2. Added how to setup Android NDK

* Version 1:
1. Added how to build a nnstreamer library
2. Added how to build a test application

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>