add login credentials to README

Author: BhumikaSaini

README.md

@@ -22,6 +22,9 @@ Building the virtual machine is this easy:
     host $ cd rails-dev-box
     host $ vagrant up
 
+(If prompted for login credentials in the **VirtualBox VM**, use:  
+**Username:** vagrant **Password:** vagrant)
+
 That's it.
 
 After the installation has finished, you can access the virtual machine with