Add CONTRIBUTING.md (#1060)

* Add CONTRIBUTING.md

* add space

.github/CONTRIBUTING.md

@@ -0,0 +1,53 @@
+# Contributing to Terraform - Google Provider
+
+For a set of general guidelines, see the [CONTRIBUTING.md](https://github.com/hashicorp/terraform/blob/master/.github/CONTRIBUTING.md) page in the main Terraform repository.
+
+The following are certain Google Provider-specific things to be aware of when contributing.
+
+## Go
+
+We aim to make the Google Provider a good steward of Go practices. See https://github.com/golang/go/wiki/CodeReviewComments for common Go mistakes that you should attempt to avoid.
+
+## Tests
+
+### Running Tests
+
+The following environment variables must be set in order to run tests:
+```
+GOOGLE_PROJECT
+GOOGLE_CREDENTIALS or GOOGLE_USE_DEFAULT_CREDENTIALS
+```
+For certain tests, the following variables may also need to be set:
+```
+GOOGLE_REGION
+GOOGLE_ORG
+GOOGLE_BILLING_ACCOUNT
+```
+
+To run a specific test, use a command such as:
+```
+make testacc TEST=./google TESTARGS='-run=TestAccContainerNodePool_basic'
+```
+
+The `TESTARGS` variable is regexp-like, so multiple tests can be run in parallel by specifying a common substring of those tests (for example, `TestAccContainerNodePool` to run all node pool tests).
+
+### Writing Tests
+
+Tests should confirm that a resource can be created, and that the resulting Terraform state has the correct values, as well as the created GCP resource.
+
+Tests should confirm that the resource works in a variety of scenarios, and not just that it can be created in a basic fashion.
+
+Resources that support update should have tests for update.
+
+Resources that are importable should have a test that confirms that every field is importable. This should be part of an existing test (in the regular resource_test.go file) as an extra TestStep with the following format:
+```
+resource.TestStep{
+	ResourceName:      "google_compute_backend_service.foobar",
+	ImportState:       true,
+	ImportStateVerify: true,
+},
+```
+
+## Autogenerated Resources
+
+Some resources in the Google provider are autogenerated using the [Magic Modules](https://github.com/GoogleCloudPlatform/magic-modules) framework, and have a large disclaimer as a comment at the top of their files to say so. For feature requests or bugs regarding those resources, please continue to file [issues](https://github.com/terraform-providers/terraform-provider-google/issues) here. PRs changing those resources will not be accepted.