If you choose to tell a woman who is in a relationship that you have feelings for her, come prepared with a 'terms of agreement' style explanation of 'what this means' to your friendship. That will make it a whole lot easier.
You need to be clear on what you want, what you are willing to risk, and what you'll settle for, and you need to make that all clear an a very nice and succinct way.
I suggest a casual: 'Look, I know you are in a relationship, and I (do/don't) expect this will change that, but I have romantic feelings towards you that I (can/can't) manage if we stay friends. Ultimately, I'd (rather/rather not) continue spending as much time together as we do, considering. Now that you are enlightened to the situation, I'm hoping you are not feeling too uncomfortable, and I'm here if you want to talk about it.'
Then, whilst, still dropping a bomb on the friendship, you're not expecting her to sort through the rubble on her own.
Good Luck!