update instructions (#101)

Author: wbrowne

README.md

@@ -11,11 +11,15 @@ The `sqlds` package is intended to remove the repetition of these datasources an
 **Usage**
 
 ```go
-ds := sqlds.NewDatasource(&myDatasource{})
-if err := datasource.Manage("my-datasource", ds.NewDatasource, datasource.ManageOpts{}); err != nil {
+if err := datasource.Manage("my-datasource", datasourceFactory, datasource.ManageOpts{}); err != nil {
   log.DefaultLogger.Error(err.Error())
   os.Exit(1)
 }
+
+func datasourceFactory(s backend.DataSourceInstanceSettings) (instancemgmt.Instance, error) {
+  ds := sqlds.NewDatasource(&myDatasource{})
+  return ds.NewDatasource(s)
+}
 ```
 
 ## Standardization