Add mwan3 to packages feed

Hi guys,

I'd like to add mwan3 to the main packages feed please. I am the maintainer of this script. Mwan3 makes load-balancing of multiple wan interfaces easy. It has been tested by multiple users. More info: https://forum.openwrt.org/viewtopic.php?id=39052

This is my first merge request. If i have done anything wrong in submitting, please point me in the right direction.

Thnx!

Singed-off-by: Jeroen Louwes <jeroen.louwes@gmail.com>