Add tags set conversion to upgrade guide (hashicorp#218)

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/cc @danawillow

website/docs/version_2_upgrade.html.markdown

@@ -468,6 +468,12 @@ Autogenerated buckets are shared by all clusters in the same region, so deleting
 this bucket could adversely harm other dataproc clusters. If you need a bucket
 that can be deleted, please create a new one and set the `staging_bucket` field.
 
+### `cluster_config.0.gce_cluster_config.0.tags` is now a Set
+
+The order of entries in `tags` no longer matters. Any configurations that
+interpolate based on an item at a specific index will need to be updated, as items
+may have been reordered.
+
 ## Resource: `google_filestore_instance`
 
 ### filestore resources have been removed from the GA provider