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>

CHANGELOG.md

@@ -16,6 +16,10 @@
   capability checks that Firecracker performs during boot. If any of
   these fields are in use, minimal target snapshot version is
   restricted to 1.5.
+- 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