Add license file

[icedream]

Add a license compatible with https://github.com/icedream/go-bsdiff/blob/master/bsdiff/LICENSE, maybe just go with the same license.

[whereswaldon]

I'd love to be able to use this in some projects, but without a license I can't. If you'd like folks to be able to use your work, we'd appreciate it if you could add a license.

[icedream]

I've now licensed it under the 2-clause BSD license (same one as the used bsdiff codebase itself).

[whereswaldon]

@icedream That's very generous; thank you for enabling others to use this!