Update README.md

Author: jackycute

README.md

@@ -48,6 +48,7 @@ DB migration
 If you are upgrading HackMD from older version, you might need to do this to apply the DB schema changes.  
 
 1. Modify the file named `.sequelizerc`, change the value of the variable `url` with your db connection string.  
+   For example: `postgres://username:password@localhost:5432/hackmd`
 2. Run `node_modules/.bin/sequelize db:migrate`, that's it!
 
 Structure