Gson customization docs

Adds a Gson customization section to the website.

Is there a way to get the built in `Gson` class to customize it? I currently provide it from an external gson jar, so that is how I wrote the documentation. I can change this if there is a way to pull it from Retrofit.

I'm not sure how detailed we want these docs to be. I figured this would be a great starting point, without adding a ton of details about adding gson to your app's libs.

This should fulfill the requirements for the first item on the Cookbook list square/retrofit#256