Adjust the dbt README to document the use of the --target flag

dbt/README.md

@@ -48,6 +48,16 @@ Build the models to create views in our Athena warehouse:
 dbt run
 ```
 
+By default, all `dbt` commands will run against the `dev` environment, which
+namespaces the resources it creates by prefixing target database names with
+your Unix `$USER` name (e.g. `jecochr-default` for the `default` database when
+`dbt` is run on Jean's machine). To instead **run commands against prod**,
+use the `--target` flag:
+
+```
+dbt run --target prod
+```
+
 Generate the documentation:
 
 ```