Add Package Manifest (package.json) for Ardity (#76)

I've added a Package Manifest (package.json) to Ardity to allow Unity developers to use Package Manager to manage it. This will make it easier to install and update Ardity, as well as manage its dependencies.

Changes:

1. Added package.json file to UnityProject/Assets/Ardity
2. Updated README.md file to include instructions on how to install Ardity using Package Manager

Author: tedliou

README.md

@@ -21,6 +21,22 @@ If you need a program to test your Unity scene I have created the following Ardu
 It also recognizes two input messages and reacts to them, this is for bidirectional communication for the scene "DemoSceneUserPoll_ReadWrite".
 So if you want to use that scene just run it and press the keys 'A' or 'Z' to see what you Arduino has to say about it.
 
+Installation
+============
+
+You can download the code from GitHub (Code > Download ZIP) and after uncompressing, import the assets into your Unity scene.
+
+
+Alternative installation using Package Manager
+============
+
+1. Open the Unity Package Manager by navigating to **Packages > Package Manager**.
+2. Click on **Packages** in the top left corner and select **Add package from git URL...**.
+3. Paste the following URL into the input field and click **Install**:
+
+```
+https://github.com/dwilches/Ardity.git?path=UnityProject/Assets/Ardity
+```
 
 Sample Arduino Program
 ======================

UnityProject/Assets/Ardity/Scripts/Editor.meta

@@ -0,0 +1,16 @@
+{
+    "name": "Ardity.Editor",
+    "rootNamespace": "",
+    "references": [],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": true,
+    "defineConstraints": [],
+    "versionDefines": [],
+    "noEngineReferences": false
+}

UnityProject/Assets/Ardity/Scripts/Editor/Ardity.Editor.asmdef

@@ -0,0 +1,3 @@
+{
+	"name": "Ardity"
+}