chore: add the license and contribution notes (#127)

Author: toku-sa-n

CHANGELOG.md

@@ -1,6 +1,9 @@
 # Changelog
 
 ## Unreleased - ReleaseDate
+### Added
+- License and contribution notes are added to the README.
+
 ### Fixed
 - Outdated code example in `README.md` is fixed.
 

README.md

@@ -12,7 +12,7 @@ Users can use this library to implement a USB device deriver on your own OS.
 
 This crate is `#![no_std]` compatible.
 
-# Examples
+## Examples
 
 ```rust
 let mut r = unsafe { xhci::Registers::new(MMIO_BASE, mapper) };
@@ -23,3 +23,20 @@ o.usbcmd.update(|u| {
 });
 while o.usbsts.read().hc_halted() {}
 ```
+
+## License
+
+Licensed under either of
+
+ * 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 the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.