Add test instructions and fix region disk test (#500)

As a step toward adding integration testing into
the CI process for this project, adding instructions
on how to run the tests.

compute_region_disk was used as a test. Fixed
the tests which was previously hard-coded to
the `graphite-playground` GCP project to work
with the project in the user-configure .ini file.

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing to the google.cloud collection
+
+## Cloning
+
+The `ansible-test` command expects that the repository is in a directory that matches it's collection,
+under a directory `ansible_collections`. Clone ensuring that hierarchy:
+
+```shell
+mkdir -p $TARGET_DIR/ansible_collections/google
+git clone <url> $TARGET_DIR/collections/google/cloud
+```
+
+## Running tests
+
+### prequisites for all tests
+
+- Install the `ansible` package.
+
+## Running integration tests
+
+### Integration testing prequisites
+
+#### Installing personal GCP credentials
+
+The integration tests for this module require the use of real GCP credentials, and must provide
+ansible-test those values. They can be added by authoring the following in `tests/integration/cloud-config-gcp.ini`:
+
+```
+[default]
+gcp_project: @PROJECT_ID
+gcp_cred_file: @CRED_FILE
+gcp_cred_kind: @CRED_KIND
+gcp_cred_email: @EMAIL
+```
+
+### Running
+
+Run `ansible-test integration`. Currently some tests are disabled as [test are being verified and added](https://github.com/ansible-collections/google.cloud/issues/499).