zstd-sys should use the BSD 3-clause license

[complexmath]

I see that the BSD 3-clause license that the zstd library uses was added to the zstd-sys tree here: #171. However, the zstd-sys LICENSE file says that the package uses the MIT or Apache licenses with no mention of the BSD license despite all the generated code in the project using the BSD license. I think the zstd-sys package should be changed to using only the BSD license to reflect this, and so the license is represented properly in the zstd-sys crate. Since the 3-clause license has a documentation requirement, it seems highly likely that with the current setup, many users of zstd are in violation of the license requirements simply from not knowing that the BSD license is involved.