I almost pre-responded to this response, since I knew it was coming.
Put it in a FAQ or even a link - There is a link to "forgot password" on the sign-in page, plus a FAQ, plus an article in the FAQ called "Having trouble logging in?", plus some smarts in the ticket system to prompt people who email about logging in with a message saying, "Maybe you should read this article titled 'Having trouble logging in?'
That's like 4 layers of tech-support between the user and me, and I still get emails about people needing password help.
Trust me: Any site with a sizable userbase will have a lot of support requests. You can find ways to mitigate some of it, but the only way to mitigate all of it is to provide poor customer support.