Romeo and Juliet is in the public domain. And it's not even the source material - Shakespeare borrowed the story from somewhere else, (Pyramus and Thysbe is one very similar story, and Ovid didn't invent it either) and retold it in the form of the famous play. That means you're free to rework the source material.
The same is true of common fairy tales, and for works that are in fact original, but already in the public domain. For example, Rudyard Kipling's The Jungle Book recently got retold by Neil Gaiman as The Graveyard Book. No mentions of the original - it's for the reader to figure it out. (Technically, he mentioned it in the Acknowledgements section, but that's not a legal requirement).
Retellings of older stories are common enough, and fall within the realm of intertextuality. That's when there is a strong relationship between two texts, and the latter is better understood in light of the former.
If you feel strongly that you should acknowledge the influence of another work on yours, you can always mention it in the Acknowledgement section in the end. But I do not recall James Joyce, for example, formally acknowledging that Ulysses is strongly connected to the Iliad. He expected readers to be smart enough to figure that out by themselves (from the title, for one thing).
Note that all this is only true for works that are in the public domain. If a work is not yet in the public domain, you can expect to beet accusations of plagiarism.