add testresult example to readme

[StefanSchoof]

Adding additional example to the local file export, as discussed in #963

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "localfileexample" git@github.com:StefanSchoof/buildkit.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[tonistiigi]

It looks a bit weird to me. First "export test result file" looks very specific. Then, for "--from=builder" I have no idea where this builder image is coming from or what it contains.

Do you just want to say: "To export specific files use multi-stage builds with a scratch stage and copy the needed files into that stage with COPY --from." (you can still have the code example)

[StefanSchoof]

@tonistiigi thanks for the better text. I changed it in PR.