Take her out for a cup of coffee and tell her that there is something you should have mentioned earlier, but you were worried about scaring her off and you really, really like her. Then tell her that you and your partner are in an open relationship. Explain how being poly works and that she by no means has to be anything other than friends with your partner. Be prepared for her to feel betrayed by you not telling her earlier or weirded out.
She might run for the hills, but get back to you a week later and you can send her some info material. She might run for the hills and just be gone. She might ask you lots of questions, so be prepared to answer them and not be offended when some of them seem unfortunate.
You might want to make up your mind as to what you envision her to be if you guys work out. A secondary? Another primary? Do you really envision yourself in a triad or do you think you could be happy in a V? Those will influence your answers and you have to be as honest as you can. You might also want to consider if you imagine that there might be a third after her or if you are really aiming for two. It might not come up, but if it does, then you want to be prepared.
Essentially - be honest, try to be prepared and do not take it hard whatever she says, as she is likely to be shocked and a little hurt.