Added Building guide

Buidling.md

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+If you wish to build the extension yourself, or you want to add code to it, this guide will help you.
+## Get the code
+You can get the code in different ways:
+- Cloning the repo via git: `git clone https://github.com/moonlight-stream/moonlight-chrome`
+- Getting the code as a zip ("Clone or download" > "Download zip" or via the releases tab, see "Source code (zip)")
+
+Once completed, you'll have a directory (here named `moonlight-chrome`) containing the code of the application. Go inside it and install the requirements.
+
+## Requirements
+* Git: https://git-scm.com/downloads
+* Python 2.7: https://www.python.org/downloads/release/python-2713/
+
+__Make sure they are properly installed__ by starting your console and typing: `git --version` and `python --version`. If no error is printed inside the console, you're good to go!
+
+## Install the Chrome Native App SDK
+> This is quoted from [the pepper.js wiki](https://github.com/google/pepper.js/wiki/Getting-Started).
+
+- Install the [NaCl SDK](https://developers.google.com/native-client/sdk/download). Set the `NACL_SDK_ROOT` environment variable to be an *absolute* path that points to the desired `pepper_*` directory inside of the NaCl SDK. For
+example, to temporarily set the environment variable:
+```shell
+set NACL_SDK_ROOT=c:\path\to\nacl_sdk\pepper_*  (Windows)
+export NACL_SDK_ROOT=/path/to/nacl_sdk/pepper_*  (Almost everything else)
+```
+> Moonlight uses the latest version of Pepper, which [you can get here](https://developer.chrome.com/native-client/sdk/download#installing-the-stable-bundle)
+
+## Compiling the NaCl module
+The compile process has been done in **Windows only** for now
+- Go to the directory where you downloaded the code: `cd moonlight-chrome` for example
+- Open up your console: _Shift+RightClick_ on Windows
+- Type in `git submodule update --init --recursive` to init git submodules required for building
+- Type in `make.bat`: the console should log some information
+- When it's done, you can move over to the final step
+
+## Adding the extension in Chrome
+- Go to `chrome://extensions`
+- Tick "Developer mode"
+- Click "Load unpacked extension"
+- Go to the directory where you installed Moonlight (select its name)
+- Moonlight should appear in the list
+
+Launch it by clicking "Launch" in the extensions list or via `chrome://apps`. If making changes, don't forget to __reload the extension__ by clicking "Reload" in chrome://extensions and running `make` if you've changed the NaCl module!
+Chrome will now remember you that you have an unpacked extension that could lead to security problems. To remove this message, we recommend you **disable the extension** when you're not using it.