fix documentation on project mode

Author: Kayrnt

docs/configuration.md

@@ -35,6 +35,17 @@ vars:
 
 To enable the "project mode", you'll need to define explicitly one mandatory setting to set in the `dbt_project.yml` file:
 
+```yml
+vars:
+  # dbt bigquery monitoring vars
+  input_gcp_projects: [ 'my-gcp-project', 'my-gcp-project-2' ]
+```
+
+##### GCP Billing export
+GCP Billing export is a feature that allows you to export your billing data to BigQuery. It allows the package to track the real cost of your queries and storage overtime.
+To enable on GCP end, you can follow the [official documentation](https://cloud.google.com/billing/docs/how-to/export-data-bigquery) to set up the export.
+Then enable the GCP billing export monitoring in the package, you'll need to define the following settings in the `dbt_project.yml` file:
+
 ```yml
 vars:
   enable_gcp_bigquery_audit_logs: true