gax docs linked in README are out of date

[leahecole]

Per our discussion in chat and today's meeting

In the README, we link to http://googleapis.github.io/gax-nodejs/index.html as the docs but this is out of date. I found https://googleapis.dev/nodejs/google-gax/latest/index.html, but I believe that googleapis.dev is also not the most up to date location we want - are these published in Cloud RAD? if so, we should update the README to reflect it!