Create a README file in the cache directory like Go does

[fhunleth]

Obviously, Go's use of the cache directory is different, but I thought that it will really nice of Go to put a short readme in it. I think we should do something similar. For reference, here's what Go's readme looks like:

$ cat ~/.cache/go-build/README
This directory holds cached build artifacts from the Go build system.
Run "go clean -cache" if the directory is getting too large.
See golang.org to learn more about Go.