Update changelog for backport 0.9.17 release

Author: phil-opp

Changelog.md

@@ -14,6 +14,10 @@
 - Rewrite for UEFI support ([#130](https://github.com/rust-osdev/bootloader/pull/130))
   - Includes a new build process that no longer uses the `bootimage` crate. See the Readme for details. 
 
+# 0.9.17 _(backport)_ – 2021-04-30
+
+- Reduce the number of used unstable features of x86_64 crate (backport [#155](https://github.com/rust-osdev/bootloader/pull/140))
+
 # 0.9.16 – 2021-03-07
 
 - Replace all remaining `lea`s with `mov` + `offset` ([#140](https://github.com/rust-osdev/bootloader/pull/140))