changelog: pmem: add a note about rate-limiter support in virtio-pmem

Mention new rate-limiter API for the virtio-pmem device in the
CHANGELOG.md

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

Author: ShadowCurse

CHANGELOG.md

@@ -14,6 +14,9 @@ and this project adheres to
   support for Vsock Unix domain socket path overriding on snapshot restore. More
   information can be found in the
   [docs](docs/vsock.md/#unix-domain-socket-renaming).
+- [#5789](https://github.com/firecracker-microvm/firecracker/pull/5789): Add
+  rate-limiter support to virtio-pmem device to allow control over I/O bandwidth
+  generated by the FLUSH requests from the guest.
 
 ### Changed