docs: update README to clarify demo mode usage and Nimiq Hub requirements

Author: onmax

README.md

@@ -10,7 +10,9 @@ yarn install
 yarn serve
 ```
 
-> You will also need to run the [Nimiq Hub](https://github.com/nimiq/hub#contribute) and [Nimiq Keyguard](https://github.com/nimiq/keyguard/#development) in development, too.
+If you need to interact with a real account and data, then you will also need to run the [Nimiq Hub](https://github.com/nimiq/hub#contribute) and [Nimiq Keyguard](https://github.com/nimiq/keyguard/#development) in development, too.
+
+Otherwise, you can use activate the `demo` mode by appending to `?demo` in the URL: `http://localhost:8081/?demo`.
 
 ### Compiles and minifies for production
 ```