Update changelog

Author: eldruin

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 [Full Changelog](https://github.com/rust-embedded/rust-spidev/compare/0.5.0...HEAD)
 
+- Implement `AsRawFd` for `Spidev` to allow access to the underlying file descriptor.
 - Updated nix to allow both version `0.22` or `0.23`.
 
 ## 0.5.0 / 2021-09-21