Add note to readme

README.md

@@ -46,6 +46,7 @@ Lastly, ensure that `GOPATH` is defined in your shell. If necessary, `GOPATH` ca
 1. `createdb vulcanize_public`
 1. `cd $GOPATH/src/github.com/vulcanize/vulcanizedb`
 1.  Run the migrations: `make migrate HOST_NAME=localhost NAME=vulcanize_public PORT=5432`
+    - There is an optional var `USER=username` if the database user is not the default user `postgres`
     - To rollback a single step: `make rollback NAME=vulcanize_public`
     - To rollback to a certain migration: `make rollback_to MIGRATION=n NAME=vulcanize_public`
     - To see status of migrations: `make migration_status NAME=vulcanize_public`