wrap readme for easy reading

(no other changes)
Amending api token section of README

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.
Reword session token introductory paragraph

Based on alphagov/vcloud-tools#187 and done in consultation with
@danielabel
Merge pull request #66 from alphagov/readme_fix_for_api_token

Readme fix for api token section