Update circle.ci readme

[mariacha]

Not all of these steps are necessary anymore, I believe. The Pantheon private/public key stuff is actually covered by a thinkshout-wide token available to all circle builds. https://github.com/thinkshout/drupal-project/blob/8.x-pantheon/.circleci/README.md

TODO:

• [x] Run through the steps in the readme for a brand new build.

• [x] Skip each of these steps:

• On Pantheon, log in as "Pantheon - ThinkShout CI Bot" (found in 1password).

• On Pantheon, add "dev-team+pantheon@thinkshout.com" as a team member to the site.

• In Circle.ci, add the PANTHEON_TOKEN associated with the "Pantheon - ThinkShout CI Bot" (1password) to the circle.ci Environment variables tab: https://circleci.com/gh/thinkshout/[YOUR PROJECT]/edit#env-vars

• On Circle, add the "Pantheon - ThinkShout CI Bot" (1password) Private-key to the circle.ci ssh permissions: https://circleci.com/gh/thinkshout/[YOUR PROJECT]/edit#ssh

• [x] See if circle works and deploys to Pantheon. If it does, remove these items. If it doesn't figure out what is actually necessary. Work from the bottom to the top, building each time until it works.

[mariacha]

@bdimaggio will you be setting up the Freedom House build? If so, this would be a handy one to do at that time.