Update credential instructions in README
Update credential instructions in README

Google changed the page recently. the endpoint is now console.developers.google.com.

And the UI is changed a little bit. Now they have `Enable API` button in dashboard. I've adjusted the instructions. Happy to modify them depending on the review.
dbs/boomerang,BruGTUG/BruGTUG.github.io,dbs/boomerang,gdg-x/boomerang,BruGTUG/BruGTUG.github.io,Splaktar/boomerang,gdg-x/boomerang,Splaktar/boomerang