CHANGELOG: Mention PVH boot mode

Firecracker now supports PVH boot as an alternative to "Linux" boot on
the x86_64 architecture.  This makes it possible for FreeBSD to boot,
and also affects how Linux kernels compiled with the CONFIG_XEN_PVH=y
option boot.

Signed-off-by: Colin Percival <cperciva@freebsd.org>

Author: cperciva

CHANGELOG.md

@@ -4,6 +4,11 @@
 
 ### Added
 
+- Added support for PVH boot mode.  This is used when an x86 kernel provides
+  the appropriate ELF Note to indicate that PVH boot mode is supported.
+  Linux kernels compiled with CONFIG_XEN_PVH=y set this ELF Note, as do
+  FreeBSD kernels.
+
 ### Changed
 
 - Updated deserialization of `bitmap` for custom CPU templates to allow usage