README.md in bigquery_migration-0.1.3 vs README.md in bigquery_migration-0.1.4

@@ -74,9 +74,20 @@
 migrator = BigqueryMigration.new(config)
 migrator.migrate_table(columns: columns)
 # migrator.migrate_table(schema_file: '/path/to/schema.json')
 ```
 
+## LIMITATIONS
+
+There are serveral limitations because of BigQuery API limitations:
+
+* Can not handle `mode: REPEATED` columns
+* Can add only `mode: NULLABLE` columns
+* Columns become `mode: NULLABLE` after type changing
+* Will be charged because a query is issued (If only adding columns, it is not charged because it uses patch_table API)
+
+This tool has an advantage that it is **faster** than reloading data entirely.
+
 ## Further Details
 
 * See [BigQueryテーブルのスキーマを変更する - sonots:blog](http://blog.livedoor.jp/sonots/archives/47294596.html) (Japanese)
 
 ## Development