doc: updated readme

Signed-off-by: skoowu <skoowu.fancy@gmail.com>

Author: skoowoo

README.md

@@ -1,5 +1,7 @@
 # Medup is a markdown parsing toolkit!
 
+![](./docs/assets/demo.png)
+
 ## Features
 * [x] Support all standard syntax of markdown
 * [x] Supports all major extension syntaxes
@@ -9,9 +11,6 @@
 * [x] Support css theme selection
 * [ ] Support git as storage backend
 
-## Demo
-todo
-
 ## Usage
 ```
 A markdown parsing toolkit