Document how to add marketplace agreement through Terraform

[kellystuard]

resolves #21

Description of the change

Updates documentation to show how to automatically accept the required marketplace agreement and create the worker pool in a single apply, by using Terraform and dependencies.

Type of change

• Bug fix (non-breaking change that fixes an issue);
• New feature (non-breaking change that adds functionality);
• Breaking change (fix or feature that would cause existing functionality to not work as expected);
• Documentation (a documentation or example fix not affecting the infrastructure managed by this module);

Checklists

Development

• All necessary variables have been defined, with defaults if applicable;
• The code is formatted properly;

Code review

• The module version is bumped accordingly;
• Spacelift tests are passing;
• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached;
• This pull request is no longer marked as "draft";
• Reviewers have been assigned;
• Changes have been reviewed by at least one other engineer;

[kellystuard]

Spacelift tests failing; seems to be due to version not being bumped. I believe since it's just a documentation change that isn't part of the release, it would be wrong for me to bump the version.