docs(readme.md): add section about backend connection, add missing general sections

Feature

Summary

This MR provides more information in the README, especially on how link the QViewer with a custom back end.

Use Cases

Everyone who wants to use the viewer apart from the Ahikar project.

Documentation

Shall we add your feature to the documentation?

• It is basically docs. :)

Changelog

• I added a statement to the CHANGELOG.

Related Tickets

Closes

Closes #26 (closed)

/cc Mathias Göbel, Frank Schneider, Michelle Weidling