I see nothing wrong with pointing out useful resources about the topic in question as a comment. In my opinion this is useful (at least more useful than silently passing by) and I also do that every now and then when I don't have time to figure out a whole answer or just know a rough direction and not enough detail.
However, the way you worded it, I can understand that some people feel slightly offended. A bit. Maybe. Okay, there are more important things in this world to be offended about, but anyway.
Let me recommend you a different wording that sounds more nice and doesn't make the OP think they are overly stupid or incompetent. What about one of those?
- Have you already read [link]? I think these resources could help you understand the problem.
- I don't have time right now, but probably you are able to figure out a solution by reading [link]. You can leave me a comment if things are still unclear to you after that and tell me what exactly you didn't understand.
- This is a pretty wide and complex topic and I think you should have at least a basic understanding of the general concepts in order to be able to understand both your problem as well as the answers and suggestions you might get. I'd like to recommend you to read [link] therefore first, please.
Those were just a few phrases how I think one could suggest somebody to read a specific resource about their problem. You don't have to copy one and learn it by heart, ready to recite it below any question, but rather be inspired to find a similar and appropriate wording yourself the next time.