docs: Readme should include links to the sample gcloud repos

[rakyll]

It'd be good to have a section that gives links to the sample app repos. For now, we should have a link to https://github.com/GoogleCloudPlatform/gcloud-node-todos on the top of the README somewhere.

[ryanseys]

I've included an examples section in the README in #244 with that application as the only entry for now.