[doc] document about thin version and upx version

需要一个解释说明各 release binary 区别的文档(备忘，近期编写)。

> In the current major release we added a lightweight version, it’s prepared for short-life container shells such as Serverless functions. We add build tags in source code and cut a few exploits to make the binary lighter, just a 2MB file, but contains 90% of CDK functions, you may also pick up useful exploits in our CDK source code to build your own lightweight binary.