Mentioning this in the job description is probably not a good idea, but only hiring female developers is your prerogative.

Specifically requesting female developers in your ad will undoubtedly get the opposite of your intention. People who are good developers who happen to be women, will not come to a company looking for female developers because they would want to be hired on merit. Those who do come with a "looking for female developers" caveat, will be subpar very likely, and/or more trouble than they are worth.

Think of it from the other side. The ad says "looking for male developers" -- what kind of men would reply to that?