Add a `CONTRIBUTING.md`

[dataders]

Describe the feature

Thanks for flagging @kyleburke-meq! We really need a CONTRIBUTING.md like other dbt repos have (e.g. dbt-snowflake's CONTRIBUTING.md.

What it should include

• the architecture of the integration_tests/ package, specifically
  • dbt_project.yml and the many if target==<DWH>
  • the tests defined in integration_tests/models/plugins/<DWH>/<DWH>_external.yml (especially YAML anchors)
• how to write a test case
• how to run tests and develop locally
  • create local environment variables based on test.env.sample
  • how to make use of integration_tests/profiles.yml
  • what commands to run
• How GitHub actions CI is set up: specifically:
  • the integration test GitHub Action .github/workflows/integration_tests.yml
  • how pull_request_target works

Who will this benefit?

Anyone who wants to contribute a to this package