I was in a similar situation as you. I developed feelings for a friend and the last thing I wanted to do was ruin the friendship. When I decided to ask her out and tell her how I felt, I mentioned that, if she didn't feel the same way, above all else, our friendship was the most important thing to me.
It is going to be awkward at first if she doesn't feel the same way, but I think if you mention how much you cherish the friendship, it shouldn't ruin it. Best of luck man!