Go into Keyboard -> Shortcuts, select "Custom Shortcuts", and press "+" to add a new shortcut.
"Name" is a descriptive name for the action (i.e. "Rotate monitor"). In "Command" type the custom command to run when the shortcut is activated.
Once the shortcut is in the list, select its row, press ENTER, then the key combination you want to activate the shortcut. If there's a conflict, the shortcut manager will tell you so, and you can choose a different combination.
You can have shortcut to enable rotated display and another to bring it back to an upright position. You can even, if you're knowledgeable enough, write a command that maintains state and just toggles between upright/rotated.
Now, as for the command you need to use, it's probably xrandr:
xrandr --output HDMI1 --rotate left
xrandr --output HDMI1 --rotate normal
The output parameter depends on which port your monitor is plugged into. To see what you currently have, type:
Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
1360x768 59.8 60.0
800x600 60.3 56.2
VGA2 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
In this case my --output would be LVDS1, as all the others are disconnected.