Provide contributor guidance on Terraform version bumps

[displague]

It would be helpful to provide steps in the CONTRIBUTING.md on bumping Terraform version dependencies under a section dedicated to this in the document.

The steps to update should include separate git commits so that we can see the manual steps distinct from the generated artifacts.

What role do the actions play in this? https://github.com/equinix/pulumi-equinix/blob/main/.github/workflows/upgrade-provider.yml

Originally posted by @displague in #75 (comment)