How does Lady Macbeth find out about the witches?Does Macbeth tell her?

In Shakespeare's Macbeth, Macbeth
tells his wife about the witches in a letter.  Act 1.5 begins with Lady Macbeth reading
the letter aloud. 

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
the letter, once she's finished reading, demonstrates
this:

Glamis
thou art, and Cawdor, and shalt be

What thou art
promised.... (Act
1.5.15-16)

She's already
determined that Macbeth shall be king as the witches
predicted. 

In addition to serving the dramatic/pragmatic
purpose of introducing Lady Macbeth and informing her of the predictions, the letter
also reveals more about Macbeth's personality.  Macbeth reveals that he "burned" with
desire to question them further--he is desperately ambitious.  And he has already made
the decision to trust them--something he wasn't at first so sure he should do.  This
will lead to his downfall.