This will setup the two extra buttons on the Logitech M570. You can map them to any key you like.
Install the proper packages.
$ yum install xvkbd xbindkeys
Create a .xbindkeysrc in your home folder with default settings.
$ xbindkeys --defaults >> ~/.xbindkeysrc
Add this to the bottom of the .xbindkeysrc you just created.
"/usr/bin/xvkbd -text "\[BackSpace]"" m:0x0 + b:8 "/usr/bin/xvkbd -text "\[Return]"" m:0x0 + b:9
Autostart xbindkeys when you login.
ln -s /usr/bin/xbindkeys ~/.kde/Autostart