chore: update CHANGELOG with network improvements

Add a note about new RX_MRGBUF feature in the virtio-net
device.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

CHANGELOG.md

@@ -10,6 +10,11 @@ and this project adheres to
 
 ### Added
 
+- [#4834](https://github.com/firecracker-microvm/firecracker/pull/4834): Add
+  RX_MRGBUF support to the `virtio-net` device. When this flag is enabled
+  guest network driver can perform better buffer management which in turn
+  improves RX and TX performance.
+
 ### Changed
 
 ### Deprecated