Add lock_table to S3 state documentation

Author: jbardin

website/source/docs/state/remote/s3.html.md

@@ -91,6 +91,8 @@ The following configuration options or environment variables are supported:
  * `secret_key` / `AWS_SECRET_ACCESS_KEY` - (Optional) AWS secret access key.
  * `kms_key_id` - (Optional) The ARN of a KMS Key to use for encrypting
    the state.
+ * `lock_table` - (Optional) The name of a DynamoDB table to use for state
+   locking. The table must have a primary key named LockID.
  * `profile` - (Optional) This is the AWS profile name as set in the
    shared credentials file.
  * `shared_credentials_file`  - (Optional) This is the path to the