Add Mate to the list of supported desktops

[Jojo144]

Hi,
Just to say that I tested keepass2-plugin-tray-icon and keepass2-plugin-application-indicator with Ubuntu Mate Vivid and both seem to work fine!
(At least, installation, minimization to tray and restoration.)
You could add Mate to the list of supported desktops :-)

I could do deeper testing if you explain me what to do.

[dlech]

Thanks. I've added MATE to the readme. If you can click on a menu item and it does something, there really isn't any more testing to do.