Add LEGAL file

[jbenet]

I'll have a file, called LEGAL, that serves as an entrypoint for understanding go-ipfs licensing, this will include a notice about vendored code.

[whyrusleeping]

@jbenet this doesnt appear to be done yet. Do we still need to do this?

[jbenet]

@whyrusleeping i think so, it can refer to LICENSE and the go style PATENTS (when it's added), and address dependencies. This could be moved to github.com/ipfs/community though as it should happen across repos.

[RichardLitt]

Closing in favor of ipfs/community#139.