Clarify wording of license information in README.
Clarify wording of license information in README.

This text historically was copied verbatim from rust-lang/rust's own README [1]
with the intention of licensing projects the same as rustc's own license, namely
a dual MIT/Apache-2.0 license. The clause about "various BSD-like licenses"
isn't actually correct for almost all projects other than rust-lang/rust and
the wording around "both" was slightly ambiguous.

This commit updates the wording to match more precisely what's in the
standard library [2], namely clarifying that there aren't any BSD-like licenses
in this repository and that the source is licensable under either license, at
your own discretion.

[1]: https://github.com/rust-lang/rust/tree/f0fe716dbcbf2363ab8f929325d32a17e51039d0#license
[2]: https://github.com/rust-lang/rust/blob/f0fe716dbcbf2363ab8f929325d32a17e51039d0/src/libstd/lib.rs#L5-L9

alexcrichton/wio