Can I include online reading guidance as research experience?
This is something I would just explain like you did in this question. From a potential supervisor's perspective, it is important to know that a PhD candidate is willing to go an extra mile and you clearly did that. Since a paper came out of this, you even have some evidence that you did not make up a nice story to impress someone, but you did actual research work. 

I would just put a line in the cv like "independent research work jointly with xxxx, resulting in publication [yyy]".