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.

[joshua]

@misterr8472 I hope you haven't been stuck for the past five months :). Just in case (and for the next reader) this may help.

https://gist.github.com/joshua/9ec2aa907b8173f6d4f0a58b3e393891

[shibumi]

@joshua do you have an example for v2, too?