hello-jni: add README and packaging metadata

hello-jni/.google/packaging.yaml

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+status:       PUBLISHED
+technologies: [Android, NDK]
+categories:   [Getting Started, Native]
+languages:    [C++, Java]
+solutions:    [Mobile]
+github:       googlesamples/android-ndk
+level:        BEGINNER
+icon:         hello-jni/screenshot.png
+license:      apache2

hello-jni/README.md

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+Hello JNI
+=========
+This sample shows how to call Native C++ code from a Android Java Activity using JNI. Note this requires a preview build of Android Studio and the experimental version Android Gradle plugin, see the [Pre-requisites](#pre-requisites) and [Getting Started](#getting-started) instructions.
+
+Pre-requisites
+--------------
+
+- [Android NDK](https://developer.android.com/ndk/)
+- Android Studio 1.3 Preview from the [canary channel](http://tools.android.com/download/studio/canary)
+- Experimental version of Android Plugin for Gradle [build from sources](http://tools.android.com/build/gradleplugin).
+
+Getting Started
+---------------
+
+1. [Download](http://tools.android.com/download/studio/canary) or [switch](http://tools.android.com/download/studio/canary) to Android Studio canary channel.
+1. Follow [instructions](http://tools.android.com/build/gradleplugin) to build the experimental version of the Android Plugin for Gradle.
+1. Launch Android Studio.
+1. Open `android-ndk/hello-jni` sample.
+1. Open *File/Project Structure...*
+  1. Click *Download* or *Select NDK location*.
+1. Click *Tools/Android/Sync Project with Gradle Files*.
+1. Click *Run/Run 'app'*.
+
+Screenshots
+-----------
+
+![screenshot](screenshot.png)
+
+Support
+-------
+
+If you've found an error in these samples, please [file an issue](https://github.com/googlesamples/android-ndk/issues/new).
+
+Patches are encouraged, and may be submitted by [forking this project](https://github.com/googlesamples/android-ndk/fork) and
+submitting a pull request through GitHub. Please see [CONTRIBUTING.md](CONTRIBUTING.md) for more details.
+
+- [Stack Overflow](http://stackoverflow.com/questions/tagged/android-ndk)
+- [Google+ Community](https://plus.google.com/communities/105153134372062985968)
+- [Android Tools Feedbacks](http://tools.android.com/feedback)
+
+License
+-------
+
+Copyright 2015 Google, Inc.
+
+Licensed to the Apache Software Foundation (ASF) under one or more contributor
+license agreements.  See the NOTICE file distributed with this work for
+additional information regarding copyright ownership.  The ASF licenses this
+file to you under the Apache License, Version 2.0 (the "License"); you may not
+use this file except in compliance with the License.  You may obtain a copy of
+the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+License for the specific language governing permissions and limitations under
+the License.