There's a lot of guidance on the regulations / design standards for how signals should be set up in a geometric sense (http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol6/section2.htm - see section 2 part 5 and part 8 although this is more about geometric design and not signal design as such) but not a lot to do with design of phasing/staging really. One of the difficulties with designing signals for roundabouts specifically is that they tend to be very difficult to configure in a good way without seeing traffic flowing across them and knowing what the traffic flows are like, and roundabouts can be vary varied in their exact layouts. It's also true that most signals now used adaptive control to adjust the timings on-the-fly. General tips that I'd give are:

• Keep the signal cycle short. In the real world cycles tend to be 45-60 seconds for roundabouts while they go up to 90-120 seconds at other junctions, depending on where in the country you are;

• Keep the circulatory clear as much as possible (provide green-waves through the busy movements);

• Provide flaring as much as you can on the entry links to get as much throughput over the stopline during each green time.

I know this is fairly difficult in Cities, even using the mods that are available. Sometimes the preferred method is to specifically hold traffic on certain entries to make sure that the rest of the junction works well, and then giving that arm more green after the busy period. This probably doesn't work so well in Cities though as there isn't really a 'peak' period of traffic.

I've just had a search online and there's this Local Transport Note that gives a lot of background on why to use signals / etc but it still doesn't really provide details on how to design the signal strategy itself: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/329062/ltn-1-09_Signal_controlled_roundabouts.pdf