Add CONTRIBUTING.md
Add CONTRIBUTING.md

Part of the open source release process.

Change-Id: I3fc76c8696c53abe37d1de6682470dd3e98b0ecf

GoogleCloudPlatform/kms-integrations,GoogleCloudPlatform/kms-integrations,GoogleCloudPlatform/kms-integrations,GoogleCloudPlatform/kms-integrations,GoogleCloudPlatform/kms-integrations,GoogleCloudPlatform/kms-integrations