README.md in bigquery_migration-0.1.0.pre1 vs README.md in bigquery_migration-0.1.0.pre2

@@ -18,9 +18,11 @@
 
     $ gem install bigquery_migration
 
 ## Usage
 
+Define your desired schema, this tool automatically detects differences with the target table, and takes core of adding columns, or dropping columns (actually, select & copy is issued), or changing types.
+
 ### CLI
 
 config.yml
 
 ```yaml