Update documentation for this repository.

What's this PR do?

This pull request updates the "getting started" documentation for this repository, since it hadn't been fully updated since we started using Terraform Cloud (and running our "applies" through their CI workflow & not from the terminal).

How should this be reviewed?

I'd recommend reviewing in the "rich" diff for 8f4c79e and 8ed1683 or viewing the "readme" on the branch!

Let me know if there's anything that needs more explanation or isn't super clear.

Any background context you want to provide?

📚 📆

Relevant tickets

N/A

Checklist

[ ] This PR has been added to the relevant Pivotal card.

DoSomething / infrastructure

Update documentation for this repository. #280

What's this PR do?

How should this be reviewed?

Any background context you want to provide?

Relevant tickets

Checklist