Build instructions for Android (#15)

Author: lmaskati

Figures/android_build.png

@@ -7,6 +7,60 @@ Node-API contract from Node.js allows consumers of this library to interact with
 JavaScript engine with a consistent interface. This library also provides some optional
 polyfills that consumers can include if required.
 
+
+## **Building - All Development Platforms**
+
+**Required Tools:** [git](https://git-scm.com/), [CMake](https://cmake.org/), [node.js](https://nodejs.org/en/)
+
+The first step for all development environments and targets is to clone this repository. 
+
+Use a git-enabled terminal to run the following command.
+
+```
+git clone https://github.com/BabylonJS/JsRuntimeHost.git
+```
+
+The unit tests require some NPM packages. From the root of the repository on the command line, run the following commands:
+
+```
+cd Tests
+npm install
+```
+
+
+## **Building on Windows, Targeting Android**
+
+_Follow the steps from [All Development Platforms](#all-development-platforms) before proceeding._
+
+**Required Tools:**
+[Android Studio](https://developer.android.com/studio), [Node.js](https://nodejs.org/en/download/), [Ninja](https://ninja-build.org/)
+
+The minimal requirement target is Android 5.0.
+
+Only building with Android Studio is supported. CMake is not used directly. Instead, Gradle
+is used for building and CMake is automatically invocated for building the native part.
+An `.apk` that can be executed on your device or simulator is the output.
+
+
+First, download the latest release of Ninja, extract the binary, and add it to your system path.
+
+Once you have Android Studio downloaded, you need to set up an Android emulator if you do not have a physical Android device. You can do this by selecting `Tools` -> `Device Manager` and then selecting a device. (We are using Pixel 2 API 27). 
+
+Open the project located at
+`JsRuntimeHost\Tests\UnitTests\Android` with Android Studio. Note that this can take a while to load. (The bottom right corner of the Android Studio window shows you what is currently being loaded.) 
+
+
+Then in the LEFT PANE, right click on `app`, and select `Run 'All Tests'`, as displayed in the image below.
+
+![Run All Tests](./Figures/android_build.png)
+
+If you don't have an Android device plugged in or no Android image in the Android emulator, that option will be greyed and inaccessible. 
+
+**Troubleshooting:**
+If the `app\cpp` folder on the left navigation pane is empty, select `File` -> `Sync Project with Gradle Files` and try to re-run the project by selecting `Run` -> `Run 'All Tests'`.
+
+Sometimes, you may need to clean the build. To do that, delete the `Debug` folder located at `JsRuntimeHost\Build\Android\Debug`.
+
 ## Contributing
 
 Please read [CONTRIBUTING.md](./CONTRIBUTING.md) for details on our code of conduct, and 
@@ -19,4 +73,4 @@ Security Response Center (MSRC) at [[email protected]](mailto:secure@microsof
 You should receive a response within 24 hours. If for some reason you do not, please 
 follow up via email to ensure we received your original message. Further information, 
 including the [MSRC PGP](https://technet.microsoft.com/en-us/security/dn606155) key, can 
-be found in the [Security TechCenter](https://technet.microsoft.com/en-us/security/default).
+be found in the [Security TechCenter](https://technet.microsoft.com/en-us/security/default).