Readme should specify that add_artifact! requires a compressed tarball

[BambOoxX]

Hello, after failing at creating an artifact with this package, I asked a question on discourse here.
The solution was that add_artifact! seems to expect a tar.gz compressed tarball and not a simple tar file.
Can you confirm this ?

If this is the case, I think it could be useful to add a specific comment on that matter so that other users don't have the same troubles that I faced.

Thanks anyway for this very helpful piece of code !

[BambOoxX]

Bump ?

[simeonschaub]

Yes, that is a limitation of the Artifact system in general. Feel free to make a PR clarifying this