Add guide for Recaptcha on sign up

[maartenvanvliet]

A guide could be added to illustrate how to add recaptcha to the sign up form. Recaptcha is a common way to add a captcha to a sign up form.

See https://github.com/plataformatec/devise/wiki/How-To:-Use-Recaptcha-with-Devise for a similar guide for Devise.

[danschultzer]

Obviously custom controllers would be used, but this could also be dealt with in a plug and devs can keep on using the default Pow controllers. The downside is that if something goes wrong, then all the filled in values will be lost. However, with reCAPTCHA we can be pretty sure that very few will get an error response since the validation already has happened client side with JS.

For this guide I would also be interested to see alternatives to reCAPTCHA so it's not just depending on one service.