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>

diff --git a/jni/README.md b/jni/README.md
new file mode 100644 (file)
index 0000000..8256ec3
--- /dev/null
+++ b/jni/README.md
@@ -0,0 +1,43 @@
+# How to build C/C++ source code with nkd-build 
+
+This manual is to describe how to generate the .so files from native C/C++
+source code. In general, these .so files are used as a low-level library
+for Android Application framework. Then, These files have been enabled by
+Android application via JNI interface.
+The libnnstreamer.so file is to be used as a native libraries on Android devices.
+ * Host PC: Ubuntu 16.04 x86_64 LTS
+ * CPU Architecture: ARM 64bit (aarch64)
+ * Android platform: 7.0 (Nougat)
+ * Android NDK: r12b
+ * Android API level: 24
+
+## Set-up Android NDK
+```bash
+cd ~/android/
+wget https://dl.google.com/android/repository/android-ndk-r12b-linux-x86_64.zip
+vi ~/.bashrc
+export ANDROID_NDK=~/android/android-ndk-r16b
+export PATH=$ANDROID_NDK:$PATH
+```
+
+## Download prebuilt Android-Gstreamer libraries
+```bash
+mkdir -p ~/android/gst_root_android/arm64
+Please download required files such as "*.tar.bz2" from http://nnsuite.mooo.com/warehouse/.
+ * gstreamer-prebuilts-for-android-device/gst_root_android-custom-1.12.4-ndkr12b-20190213-0900/
+vi ~/.bashrc
+export GSTREAMER_ROOT_ANDROID=~/android/gst_root_android
+```
+
+## How to build a NNstreamer library
+```bash
+cd ./jni
+rm -rf ../libs/ ../obj/ ; ndk-build APP_BUILD_SCRIPT=./Android.mk -j$(nproc)
+ls -al ../libs/arm64-v8a/libnnstreamer.so
+```
+
+## How to build a test application
+```bash
+$ ndk-build APP_BUILD_SCRIPT=./Android-app.mk
+ls -al ../libs/arm64-v8a/
+```