Added a demo.

Author: ynn

README.md

@@ -6,6 +6,10 @@ As a vtuber streams a live with his/her virtual appearance, Coefontuber lets you
 
 It is written in Go.
 
+| [![](./readme_assets/demo.png)](https://www.youtube.com/watch?v=e4FaFSA-_7g) |
+|:-:|
+| Demo |
+
 # 1. How It Works
 
 Say you want to make Coefontuber speak "hello", "world" and "bye". Here is what is happening under the hood:
@@ -24,6 +28,8 @@ Say you want to make Coefontuber speak "hello", "world" and "bye". Here is what
 
 # 2. Prerequisites
 
+## 2.1 Sign Up
+
 1. Sign up for [CoeFont](https://coefont.cloud/).
 
 2. Visit [https://coefont.cloud/account/api](https://coefont.cloud/account/api) to generate a key for CoeFont API.