Improve main README with image and more up to date text (#1258)

We basically haven't touched the README in a long time, which is a shame
as for many people, that will be the first thing they see about Tobira
(especially devs!). This change is nothing drastic, but it adds some
useful information and an image, which makes the readme look nicer in my
opinion.

You can preview it
[here](https://github.com/elan-ev/tobira/blob/5b011f1ab3aeb23ef88eab95aa76167a367f6891/README.md).

README.md

@@ -4,18 +4,35 @@
 ![License](https://img.shields.io/github/license/elan-ev/tobira)
 ![Latest release](https://img.shields.io/github/v/release/elan-ev/tobira?label=latest%20release)
 
-Tobira is a video portal for [Opencast](https://opencast.org).
-It provides a hierarchical page structure, with each page consisting of simple configurable content blocks (e.g. text, videos or series).
-Opencast content (series or single events) can be shown on these pages.
-Users can upload, edit (via external editor) and share their videos.
+Tobira is a video portal for [Opencast](https://opencast.org) and aims to be a pleasant interface through which users interact with your Opencast content.
+It lets you present videos, series and playlists in a customizable, hierarchical page structure, but also makes it easy for users to search through all media.
+Additionally, it offers tools to upload and manage videos.
+It's possible to connect Tobira to virtually any authentication system and integrate it into your university's/organization's infrastructure.
 
-The current version of our main branch is deployed at https://tobira.opencast.org.
-This is just a test deployment and all data is wiped whenever it is re-deployed.
-The test data was kindly provided by the ETH only for the purpose of this test deployment.
+<p align="center">
+    <img src=".github/readme-image.avif" width="95%"></img>
+</p>
+
+You can try it out for yourself here: https://tobira.opencast.org.
+This is a test deployment of the `main` branch, where most data is wiped whenever it is re-deployed.
 
 ## Documentation
 
-[**Tobira's documentation**](https://elan-ev.github.io/tobira) (both, for people trying to install Tobira and devs trying to work on Tobira's code).
+All our documentation lives here: [**Tobira's documentation**](https://elan-ev.github.io/tobira).
+Among other things, it explains how to install and configure Tobira on your own server.
+
+
+## Contribute
+
+In short: clone this repository, run the following commands and then open http://localhost:8030/.
+
+```bash
+./x.sh containers start
+./x.sh start
+```
+
+But please see [our developer documentation](https://elan-ev.github.io/tobira/dev) for more information.
+
 
 ### Name