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 a different wording that sounds nicer and doesn't make the OP feel you're implying that they are stupid or incompetent. What about one of these?

- Have you already read [link]? I think these resources could help you understand the problem.
- I don't have time to answer in detail right now, but you will probably be 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 and the answers and suggestions you might get. I'd therefore like to recommend you read [link] first, please.

Those were just a few phrases I think one could use to suggest somebody 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.