Remove Experimental marker and beta reference from clustered table

Author: potato2003

README.md

@@ -108,7 +108,7 @@ Following options are same as [bq command-line tools](https://cloud.google.com/b
 |  time_partitioning.expiration_ms  | int      | optional  | nil     | Number of milliseconds for which to keep the storage for a partition. |
 |  time_partitioning.field          | string   | optional  | nil     | `DATE` or `TIMESTAMP` column used for partitioning |
 |  time_partitioning.require_partition_filter | boolean      | optional  | nil     | If true, valid partition filter is required when query |
-|  clustering                       | hash     | optional  | nil     | (Experimental) Currently, clustering is supported for partitioned tables, so must be used with `time_partitioning` option. NOTE: **clustered tables** is a beta release. See [clustered tables](https://cloud.google.com/bigquery/docs/clustered-tables) |
+|  clustering                       | hash     | optional  | nil     | Currently, clustering is supported for partitioned tables, so must be used with `time_partitioning` option. See [clustered tables](https://cloud.google.com/bigquery/docs/clustered-tables) |
 |  clustering.fields                | array    | required  | nil     | One or more fields on which data should be clustered. The order of the specified columns determines the sort order of the data. |
 |  schema_update_options            | array    | optional  | nil     | (Experimental) List of `ALLOW_FIELD_ADDITION` or `ALLOW_FIELD_RELAXATION` or both. See [jobs#configuration.load.schemaUpdateOptions](https://cloud.google.com/bigquery/docs/reference/v2/jobs#configuration.load.schemaUpdateOptions). NOTE for the current status: `schema_update_options` does not work for `copy` job, that is, is not effective for most of modes such as `append`, `replace` and `replace_backup`. `delete_in_advance` deletes origin table so does not need to update schema. Only `append_direct` can utilize schema update. |