Update README.md

README.md

@@ -303,7 +303,7 @@ Contains a managed pointer to the packet.
 
 `Packet.HasReferences` checks references to the packet.
 
-`Packet.SetFreeCallback(PacketFreeCallback callback)` set callback to notify the programmer when an appropriate packet is being destroyed.
+`Packet.SetFreeCallback(PacketFreeCallback callback)` set callback to notify the programmer when an appropriate packet is being destroyed. Pointer `IntPtr` to a callback can be used instead of a reference.
 
 `Packet.Create(byte[] data, int offset, int length, PacketFlags flags)` creates a packet that may be sent to a peer. The offset, length and packet flags parameters are optional. Multiple flags can be specified at once. Pointer `IntPtr` to a native buffer can be used instead of a reference to a byte array.