She begins the scene reading the word, "They," which is a pronoun that the reader understands refers to the witches. This means that she begins reading the letter aloud--and the scene opens--after she has already read some of the letter. Macbeth, presumably, tells her about the witches in the unread portion of the letter.
He also tells her more about the witches in the letter, which she continues to read aloud. The witches, however, are not the main topic of the letter--the predictions they made and the coming true of one of them are what the letter is really about. And Lady Macbeth's response to