Update credentials docs for vcloud-login

Extracted from vcloud-core's README. With the additional removal of the
password NB, now that core is issuing deprecation warnings. I'll then point
the READMEs of core and the other projects to this page instead of
duplicating the credentials documentation.
Detail name of env var that vcloud-login exports.
Merge pull request #197 from gds-operations/vcloud_login_docs

Update credentials docs for vcloud-login