Add comment about re-captcha key with install message

[espaan]

Hi, I think it is good to display an install message about the user having to add re-captcha keys for the module to operate. Maybe also remark that adding it as a hook to modules you would like to use it.

[ghost]

It might be worth putting that in the settings admin, with a hyper link of where to get one. You're not going to get any install messages with the disro... and a flash is not really a good place to put it anyway.

[craigh]

PRs welcome.