Option to add a custom text to the create room dialog

### Your use case

#### What would you like to do?

I want to display a custom text in the create room dialog.

#### Why would you like to do it?

Sometimes organisations want to follow a specific room naming schema.
It would be good to display a note about room names in the create dialog.


#### How would you like to achieve it?

There can be a setting, that contains the message.
It can be displayed by the clients.

### Have you considered any alternatives?

No

### Additional context

None

### Are you willing to provide a PR?

No