Improve README.md

[liefersfl]

Checklist

• I have filled out the template to the best of my ability.
• This only contains 1 feature request (if you have multiple feature requests, open one feature request for each feature request).
• This issue is not a duplicate feature request of previous feature requests.

Is your feature request related to a problem? Please describe.

I was not sure which user / password i should enter. Also the rscp password i set did not work

Describe the solution you'd like

Please add following information to the README.md:

• Which username / password one should enter
• Which rscp password characters are allowed to use

Describe alternatives you've considered

Improve the error message in the ha dialog and suggest solutions like changing the rscp password

Additional context

[torbennehmer]

see also #27