unity3D发布Android版本app，无法响应返回键的解决办法

天朝一直秉承着优良传统，百度出来的东西只告诉了代码怎么实现，完全没告诉脚本如何运行，对于我这样的u3D初学者很头疼。

google了一下，搜到官网解决办法：

1. Right click on the 'Assets' section in the Project tab. Then select 'Create' > 'C# Script'.
2. A new script will be added to your 'Assets'. Rename it to a more meaningful file.
3. Double click on that file and the script editor will open. Paste the specified code into the update method. Save and exit the editor.
4. Click on the 'ImageTarget' element in the 'Hierarchy' tab.
5. Click on the 'Add Component' button available in the 'Inspector'. Select 'Script' and then choose the created file.
6. The script will be added to the component. That's all...