Add simple example of authentication and performing simple operation with go client

As you can tell from the subject line, this isn't a bug, it's more of a request for a simple example of authentication via Go.

I have found that as a somewhat less experienced developer, I have no idea where to start using the go client.

I am using porcelain. I created an API access token for my Netlify account, but now I am stuck. After two days of experimenting, I have still not found a way to do something as simple as list the sites in my account.

It would be great if the readme.md could include a simple auth example and one call to listsites or something similar.

Start with README.md and the Go client usage described in the issue, then identify the authentication setup for a Netlify API access token and the call that lists sites. The documentation is done when a newcomer can follow the example to authenticate and list the sites in their account.