Update README.md

In this plugin, a library named libVTMIOSSDK.a should be import the project, but it is larger than 100M, and was refused by github(See http://git.io/iEPt8g for more information.), so we can't commit it to github.but you can get it from the following link, and import it to your project manually.

http://pan.baidu.com/s/1hqxghuO