Incomplete documentation for terraform_docs.sh dependancy

[Dev25]

As part of the migration to Terraform 0.12 #177, eb0a0e7#diff-e9cfabf9b162cebf5922498a77b8f766R103 introduced a depedancy on a file from https://github.com/antonbabenko/pre-commit-terraform which is not mentioned in the README.

Trying to run make generate_docs on master without the script in your $PATH will lead to errors, terraform_docs.sh is required because upstream terraform-docs does not support Terraform 0.12.

[morgante]

FYI we're planning to standardize on using Docker images to run all tests, which should solve this.

[aaron-lane]

This will be fixed by #251.

[aaron-lane]

Fixed by #262.