add missing usage.json to v5 sdk tag (Azure#13960)

Author: ericshape

specification/sql/resource-manager/readme.md

@@ -51,6 +51,7 @@ input-file:
 - Microsoft.Sql/stable/2014-04-01/serviceObjectives.json
 - Microsoft.Sql/stable/2014-04-01/sql.core_legacy.json
 - Microsoft.Sql/stable/2014-04-01/usages_legacy.json
+- Microsoft.Sql/preview/2018-06-01-preview/usages.json
 - ./Microsoft.Sql/preview/2020-11-01-preview/BackupShortTermRetentionPolicies.json
 - ./Microsoft.Sql/preview/2020-11-01-preview/BlobAuditing.json
 - ./Microsoft.Sql/preview/2020-11-01-preview/DatabaseAdvisors.json