how to create new virtual sensor type?

[manjh]

Not sure where to post this question, so...

For a while now, I have been looking for a way to display some values in a virtual sensor.
As an example, I would like to have a custom sensor display one value from a remote ESP8266 unit (this is already possible, have many of these now). But I would also like to display a second value as URL.
This way, I can look at the overview of all ESP units, and if something needs action I can click the URL and get into the ESP.

I don't think this can be done using the existing virtual sensor types.
But how do I start creating one of my own? I looked at the Github source, but can't find the place to start...