Readme fix for api token section

The section of the README that covered using the VCloud API token was not complete. I've added additional details that should make it easier to use.

I've made the changes suggested in https://github.com/alphagov/vcloud-walker/pull/62, and made a new pull request as I think I was confusing things by continuing there.  (Happy to do something different)

/cc @philandstuff @annashipman @mattbostock