export auth to FAQ on wiki

RE: #182

**This PR depends on us using a Wiki for FAQ & Examples.**

For the full experience, see:

- https://github.com/stephenplusplus/gcloud-node/tree/spp--auth-faq
- https://github.com/stephenplusplus/gcloud-node/wiki/Frequently-Asked-Questions
- https://github.com/stephenplusplus/gcloud-node/wiki/Examples
I'd agree with that.