I think, this text is already built-in by default. When you typed your message on our board, you should see sentense "You can post anonymously, entering just a nickname without password, if username is not taken by another member; or leaving both fields empty." under the login fields. If it would be disallowed for anonymous to post, you should see a message "Only registered users can post here. Enter your login/password correctly before posting a message, or register first."
As Ivan wrote, these sentenses may be changed in language pack ($l_anonAllowed and $l_anonDisallowed variables).
We think it's enough for users to know what they should do; anyway, if somebody posts anonymously, he will not lose his posting anyway, because the next screen will report a warning and display his message. This works as described in newest version of miniBB, of course, and we have no plans to change something regarding that.