Edit your question, then add a comment to the answer(s) you are responding to. The former keeps the question coherent for future readers, while the latter provides a means for those providing answers to notice (via their "responses" page) and perhaps edit their existing answers.
Do not provide additional information by posting an "answer". Doing so effectively splits your question into segments that are not associated by the site, and will likely be missed both by future readers and those who have already provided answers.
Incidentally, you may notice a practice in some older questions where answers form a sort of conversation between the person asking the original question and those responding. These generally date back to the early beta period before the comment feature was introduced - it should be avoided now.