signal: 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

README.md

@@ -45,10 +45,17 @@ fn main() {
 
 # License
 
-`tokio-signal` is primarily distributed under the terms of both the MIT
-license and the Apache License (Version 2.0), with portions covered by various
-BSD-like licenses.
+This project is licensed under either of
 
-See LICENSE-APACHE, and LICENSE-MIT for details.
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+   http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+   http://opensource.org/licenses/MIT)
 
+at your option.
 
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Serde by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.