Explain custom theme

I switched to ```angular-re-captcha```, because it had full support for reCaptcha options. This allowed me to take advantage of reCaptcha's custom themes.

I thought I would share the basic setup in the readme. The most important step is adding the ```re-captcha``` directive to the root widget element. Google's documentation can get developers the rest of the way there.
https://developers.google.com/recaptcha/docs/customization

Here is an example of using it to make reCaptcha responsive:
https://github.com/stellar/stellar-client/pull/1044