add api doc for readme
add api doc for readme

BenBBear/cordova-plugin-leanpush,xfstudio/cordova-plugin-leanpush,easychen/cordova-plugin-leanpush,easychen/cordova-plugin-leanpush,xfstudio/cordova-plugin-leanpush,xfstudio/cordova-plugin-leanpush,easychen/cordova-plugin-leanpush,BenBBear/cordova-plugin-leanpush