docs: add steps for testing the connector (#582)

Author: nancynh

CONTRIBUTING.md

@@ -26,3 +26,15 @@ information on using pull requests.
 
 This project follows [Google's Open Source Community
 Guidelines](https://opensource.google/conduct/).
+
+## Testing
+
+NOTE: Be sure to run the following commands in the same VPC as the AlloyDB instance
+
+1. Set the environment variables. You can see an example of the environment
+variables needed by running `cat .envrc.example`
+1. Run `gcloud auth application-default login`
+1. Run the unit tests: `mvn test -B -ntp -Pcoverage -Dclirr.skip=true
+-Denforcer.skip=true`
+1. Run the integration tests: `mvn -ntp -Penable-integration-tests
+-DtrimStackTrace=false -Dclirr.skip=true -Denforcer.skip=true -fae verify`