Add doc.

Author: jcs090218

README.md

@@ -4,9 +4,19 @@
 # better-scroll
 > Improve user experience when scrolling window.
 
+## Usage
+
+```el
+(define-key global-map (kbd "<prior>") #better-scroll-down)
+(define-key global-map (kbd "<next>") #better-scroll-up)
+
+(define-key global-map (kbd "S-<prior>") #better-scroll-down-other-window)
+(define-key global-map (kbd "S-<next>") #better-scroll-up-other-window)
+```
+
 ## Contribution
 
-If you would like to contribute to this project, you may either 
-clone and make pull requests to this repository. Or you can 
-clone the project and establish your own branch of this tool. 
+If you would like to contribute to this project, you may either
+clone and make pull requests to this repository. Or you can
+clone the project and establish your own branch of this tool.
 Any methods are welcome!