add LICENSE-{MIT, APACHE}

[nikomatsakis]

Following the precedent of the Rust reference, Rust book, and
RFC 2044, we adopt the LICENSE-{MIT,APACHE} dual licensing
scheme for the unsafe code guidelines as well.

Fixes #20

[asajeffrey]

This can be merged once @avadacatavra, @matklad and @RalfJung agree to the relicensing over in issue #20.

[strega-nil]

Please don't use the MIT and Apache licenses for documentation. The logic for RFCs using MIT/Apache does not make sense for stuff like this. There's a reason all GNU documentation is licensed under the GNU Free Documentation License.

[asajeffrey]

@usban there's not much we can do about this, we'd like to at least keep open the option to merge with the language spec, which is MIT/Apache.

[nikomatsakis]

What @alercah said on Zulip is the clincher argument for me:

As a practical matter, even if we could put all documentation under a more appropriate license, it'd cause no end of headache if we want to move docs between rustdoc and other documentation.

"You need to put attribution in the docs for this function because it came from the book."

[nikomatsakis]

OK, going to merge this.