Added instructions for installation for Terraform 0.13.

[kovukono]

Prerequisites

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• This pull request fixes a bug.
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Description

Updates install instructions for Terraform 0.13.

Issues Resolved

List any existing issues this pull request resolves.

Checklist

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Tests

• I have added tests to cover my changes.
• All tests pass when I run make test.

Documentation

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[khos2ow]

I don't think this would just work as is with Terraform v0.13.x as the provider needs to get rebuilt after go.mod was updated (basically this https://github.com/cloud-ca/terraform-provider-cloudca/blob/master/go.mod#L5)

[kovukono]

I've been able to verify that the environment, vpc, network, ssh_key, public_ip, and instance of release 1.5.0 are currently working with Terraform 0.13.4. I could add the change to rebuild it for v1.6.0 if you'd prefer. Also, hi Khosrow!

[khos2ow]

Hi! 👋

Interesting, I didn't know that would even work without the upgrade! I think eventually it needs to get upgraded, but if it works as is maybe not very urgent.