Add Instructions to README.md Template for Terraform Pinning

[osterman]

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what

• Add these instructions: https://github.com/cloudposse/terraform-aws-tfstate-backend/issues/14#issuecomment-471413744

why

• Users are pinning to master b/c that's what our examples show
• This is a bad practice that we don't ourselves use or recommend

[sudoforge]

It would be easy enough to substitute master for tags/{{ tag_version }} in documentation. Users who are copypasting examples should get an error when Terraform attempts to clone the module if they don't change it to a valid value.