Remove deprecated "RAF" term

The README.md contains the following phrase:

> `stage1/opt/stage2` are copies of the RAFs

The term "RAF" isn't defined in the README or any other documentation files. Two code files mention it (app-container/aci/aci.go and path/path.go) but also don't define it.

I'm assuming it means Rocket Application Filesystem because path/path.go says RAF is equivalent to an app image, but I'm not sure.

The README should define RAF or just call it an "app image."