Add brief README about integration tests

integration/README.md

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+# Integration tests
+
+This directory contains integration tests for the project.
+
+The tree structure generally mirrors the source code tree structure.
+
+Requirements for new files in this directory:
+* Every integration test file **must** use the `integration` build tag
+* Every package **must** be named after its directory with `_integration` appended
+  * Requiring a different package name for integration tests avoids aliasing with the main package.
+
+## Running integration tests
+
+Integration tests require the following environment variables:
+* `CLOUD_ENV` - set to the cloud environment to use (e.g. `aws`, `azure`, `gcp`)
+* `DATABRICKS_HOST` - set to the Databricks workspace to use
+* `DATABRICKS_TOKEN` - set to the Databricks token to use
+
+Optional environment variables:
+* `TEST_DEFAULT_WAREHOUSE_ID` - set to the default warehouse ID to use
+* `TEST_METASTORE_ID` - set to the metastore ID to use
+* `TEST_INSTANCE_POOL_ID` - set to the instance pool ID to use
+* `TEST_BRICKS_CLUSTER_ID` - set to the cluster ID to use
+
+To run all integration tests, use the following command:
+
+```bash
+go test -tags=integration ./...
+```
+
+Alternatively:
+
+```bash
+make integration
+```