Update scenario doc
Add doc of default username and password for admin dashboard.
# Scenarios
## Admin
+After start up, Cello provides a dashboar for administrators, which listens on localhost:8080.
+
+The default login user name and password are `admin:pass`, you can modify this by changing the variables `USERNAME` and `PASSWORD` in the `nginx` section of the [docker-compose file](../docker-compose.yml).
### Add/Delete a host
