Update dashboard README

[elboyran]

After the redesign discussion #791 , we should change the dashboard README.
For example, the first paragraph can be changed to

The DIANNA dashboard can be used for explanation of the behaviour of several ONNX models trained for the tasks and datasets presented in the DIANNA Tutorials. The dashboard shows the visual explanation of a models' decision on a selected data item by a selected XAI method (explainer). It allows you to compare the results of different explainers, as well as explanations of the top ranked predicted labels. The dashboard was created using streamlit.

[elboyran]

Might be defendant on the outcome of #816.