docs(readme): improve slogan

Author: WittBulter

README.md

@@ -1,4 +1,11 @@
+<div align="center" height="500">
+<img src="logo.png" width="350" height="220" align="center">
+</div>
+
+<br><br>
 ### back-loader
+> a easy and steady preload lib, you can use it to load all kinds of resources. use **back-loader** in your project,
+> can quickly preload more pictures, styles(.css) or scripts, it will make your website more fluent.
 
 
 #### Usage