docs(readme): talk about dashboard user management

[petermetz]

I tried to execute the steps in the readme to bring up a test environment by running
make local and it worked fine, all the services are up and I can access the dashboard on localhost:8081 but the problem is that it is asking for user credentials that I don't have and also not sure how to register because the validation on the form doesn't seem to be happy with what I entered (see screenshot below).

[yeasy]

@xichen1 we should have mentioned the default login credential?

[petermetz]

@xichen1 we should have mentioned the default login credential?

@yeasy That would be helpful for sure! Maybe it's already documented at some obvious place and I just couldn't find it, apologies if this is the case. :-)

[xichen1]

@xichen1 we should have mentioned the default login credential?

I'll check it and put it in a clear position.

Also the signup page can be improved by showing required format if user input format is incorrect.