Update README in line with restrictions to Container passwords

RE: https://github.com/ibm-messaging/mq-container/blob/master/docs/developer-config.md#using-secrets-to-set-passwords-for-app--admin-users


The container run instructions in the main readme in this repo needs to be updated to reflect:

- Both App and Admin passwords need to be specified.

- Passing passwords as environment settings is deprecated, in favour of using secrets. 

