Update contributing docs for running integration tests (#638)

To run the integration tests locally without errors, developers have to
set more environment variables than the ones that are currently in the
docs. If these were excluded, they would get errors like:

```shell
tests/test_example_dags_no_connections.py:59: in <module>
    @pytest.mark.parametrize("dag_id", get_dag_ids())
tests/test_example_dags_no_connections.py:54: in get_dag_ids
    dag_bag = get_dag_bag()
tests/test_example_dags_no_connections.py:49: in get_dag_bag
    assert not db.import_errors
E   AssertionError: assert not {'/Users/justin.bandoro/astronomer-cosmos/dev/dags/example_cosmos_python_models.py': 'Traceback (most recent call last...ema.yml from project jaffle_shop: Parsing Error\n    Env var required but not provided: \'DATABRICKS_CLUSTER_ID\'\n\n'}
E    +  where {'/Users/justin.bandoro/astronomer-cosmos/dev/dags/example_cosmos_python_models.py': 'Traceback (most recent call last...ema.yml from project jaffle_shop: Parsing Error\n    Env var required but not provided: \'DATABRICKS_CLUSTER_ID\'\n\n'} = <airflow.models.dagbag.DagBag object at 0x1579cd000>.import_errors
```

(cherry picked from commit 156e51d)

docs/contributing.rst

@@ -111,6 +111,16 @@ To run the integration tests for the first time, use:
 
     export AIRFLOW_HOME=`pwd`
     export AIRFLOW_CONN_AIRFLOW_DB=postgres://postgres:[email protected]:5432/postgres
+    export DATABRICKS_HOST=''
+    export DATABRICKS_TOKEN=''
+    export DATABRICKS_WAREHOUSE_ID=''
+    export DATABRICKS_CLUSTER_ID=''
+    export POSTGRES_PORT=5432
+    export POSTGRES_SCHEMA=public
+    export POSTGRES_DB=postgres
+    export POSTGRES_PASSWORD=postgres
+    export POSTGRES_USER=postgres
+    export POSTGRES_HOST=localhost
     hatch run tests.py3.8-2.5:test-integration-setup
     hatch run tests.py3.8-2.5:test-integration