docs: Mention install newtonsoft json

Author: jcs090218

README.md

@@ -61,7 +61,7 @@ directory at the root of the project directory.
 These are libraries `JCSUnity` uses. Consider the usage, some plugins aren't necessary
 depend on the game your are making.
 
-- [Newtonsoft Json](https://www.newtonsoft.com/json) (required) - Popular high-performance JSON framework for .NET.
+- [Newtonsoft Json](https://www.newtonsoft.com/json) (required) - Popular high-performance JSON framework for .NET. ([`com.unity.nuget.newtonsoft-json`](https://docs.unity3d.com/Packages/[email protected]/manual/index.html); see [Newtonsoft.Json-for-Unity/wiki](https://github.com/applejag/Newtonsoft.Json-for-Unity/wiki))
 - [MyBox](https://github.com/Deadcows/MyBox) (required) - is a set of attributes, tools and extensions for Unity
 - [Tweener](https://github.com/PeterVuorela/Tweener) (required) - Simpler and light weight tweener library.
 - [In-game Debug Console](https://assetstore.unity.com/packages/tools/gui/in-game-debug-console-68068) (optional) - Easier debugging after built.