Add @wiserain as the pikpak backend maintainer

MAINTAINERS.md

@@ -16,6 +16,7 @@ Current active maintainers of rclone are:
 | Max Sum          | @Max-Sum          | union backend                |
 | Fred             | @creativeprojects | seafile backend              |
 | Caleb Case       | @calebcase        | storj backend                |
+| wiserain         | @wiserain         | pikpak backend               |
 
 **This is a work in progress Draft**