update README.md to show how to driectly go to the mysql prompt within the mysql container (laradock#592)

Author: iannazzi

README.md

@@ -534,6 +534,12 @@ docker-compose exec {container-name} bash
 docker-compose exec mysql bash
 ```
 
+*Example: enter to mysql prompt within MySQL container*
+
+```bash
+docker-compose exec mysql mysql -u homestead -psecret
+```
+
 3 - To exit a container, type `exit`.