* Add docs about local provider to CONTRIBUTING
Add docs about using a local provider with Terraform to CONTRIBUTING.md.
I couldn't find any prescriptive docs for this, so let's make them. Let me know if a different method is preferred (such as `.terraformrc`), this seemed like the best way from my brief search.
Upstreamed https://github.com/hashicorp/terraform-website/pull/474 as well to help improve the central Terraform docs on this subject.
* Update CONTRIBUTING.md (#2086)
I've seen questions about how to vendor come up from new members of our team and from the community in some OSS issues. Let's make our tribal knowledge a documented process!
