Long while ago I read it from the BK that purpose behind a warning letter is not for the maid to read as it appears to be, but instead for any future adjudicator's judgement if it so happens that disputes may arise.
The maid doesn't need to sign for agreeing to the contents of warning letter. But its necessary that there is proof of her receipt of the letter. So, either get her to sign or record it. Or, if she refuses to sign, wait until the time she acknowledges receipt of wage pay of the month, with a note next to the wage receipt saying this is acknowledgement of wage receipt together with warning letter dated XX/XX/XXXX, both received in a go.