Ignore override files as they are usually used locally
to override resources or variables without changing the code
More info: https://www.terraform.io/docs/configuration/override.html

`tfvars` files should not be gitignore-d as they are used to parameterize a Terraform root for multiple environments, e.g.:

```bash
$ tree terraform/roots/vpc
terraform/roots/vpc
├── env
│   ├── production.tfvars
│   ├── qa.tfvars
│   └── staging.tfvars
└── main.tf
```

There may be a use case where a particular `tfvars` file should be ignored, but there's no naming convention I'm aware of to easily designate such a file.

Improvements: 

 * `.terraform` directories could be created in any subdirectory when user runs `terraform init`.
 * Quite an interesting variations of`.tfstate` files are created by Terraform such as `.*.tfstate.d` or  `.*.tfstate.lock.info`. It's best not to name any file with `*.tfstate.*` pattern.
 * `*.tfvars` files often contain private data and not supposed to be committed.

Added .tfvars file so for example AWS credentials aren't committed.

See <https://www.terraform.io/docs/commands/get.html>