Add example image of topic map indicator.

[marioa]

The Topic template has a bit of text:

This is a consistent image that is found in all topics and is based on a common image representing the research cycle. The current topic is to be highlighted in the image to be used for this topic.

Would be good have an example, e.g. colour in the topic node being discussed in yellow or red so the people know what to do. It also gives a common way.

[aleimba]

Related to #25 and #67