This is great and a very acceptable goal.
> However, is it valid or proper to look specifically for female developers?
Yes, it is, but...
> Is it ok to be mention in the job ad?
No. Absolutely not.
Try to reframe your thoughts here. You're not looking for a female developer because she is a woman. You're looking for a female developer because it will give your team a set of viewpoints and skills that aren't necessarily provided by a man.
Focus on her skills and the benefits she brings to the team, rather than on her gender. You need to find a female dev that contributes as much as, if not more than, any other member of your team (regardless of gender). Hiring a good dev that's female over a great dev that's male is not only discrimination, but also a poor business choice.
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?
i.e. what languages, frameworks, libraries, etc.
Or no? By 'diversity' you just mean girls? Why do you want girls?
You also can't hire or not hire somebody because of those things you mentioned by the way.
[1]Interesting discussion on words that alienate people from underrepresented communities: https://storify.com/kissane/job-listings-that-don-t-alienate
[2] You didn't mention your location, many online groups are global and may have members in your area. Most have mailing lists, so their admins can post your job ad. The "An incomplete list of women's tech communities" section of this site lists over 30 female tech groups: http://www.hiremorewomenintech.com/
[3] There are many related tech groups on Facebook too. You can search and message their admins about posting your ad. I also recommend searching and posting related questions on Twitter.
* Reach out to women specifically and ask them to apply
* Hire diversity experts to help you (e.g. Ashe Dryden)
* Describe your company and its benefits in ways that women value (or add benefits that women value) -- talk to women to find out what that might be.
* Look critically at yourself and see if there are things about your company that make women not want to work there and change those things.
* Give yourself enough time to find women applicants.
> Any examples of job descriptions? Pro-tip: You won't find tech girls by mentioning it in job ads. *You must go out and find/meet/attract them in person. This is a hyper-competitive sport.
Here's an article on Google's quest to get more women in tech > http://www.cnn.com/2013/05/24/tech/innovation/women-at-i-o/
Ping me if you need help.