Please include a LICENSE file in the signature crate

[eclipseo]

It is required for the Fedora packaging guidelines, and would help other packagers too. It would help if you do that for other crates too.

[tarcieri]

Added in #767.

A quick spot check of this repo shows all of the other crates have licenses, and generally we do try to add them to every crate.

If you spot any other crates in other repos which don't have them, please let us know.