added contents table to readme

Author: Abhinandan707

QLever.png

@@ -1,5 +1,12 @@
 # Guidelines for creating and hosting Knowledge Graphs
 
-* [Guidelines](https://kgi4nfdi.github.io/Guidelines/)
-   * [Apache Jena](https://kgi4nfdi.github.io/Guidelines/guide/apachejena/)
-   * [Virtouso](https://kgi4nfdi.github.io/Guidelines/guide/virtuoso/)
+
+## Contents
+
+| [Guidelines](https://kgi4nfdi.github.io/Guidelines/) |  |
+|----------|----------|
+| ![Apache Jena](apache_jena.png)   | [Apache Jena](https://kgi4nfdi.github.io/Guidelines/guide/apachejena/)    |
+| ![Virtouso](virtuoso.png)   | [Virtouso](https://kgi4nfdi.github.io/Guidelines/guide/virtuoso/)    |
+| ![QLever](QLever.png)   | [QLever](https://kgi4nfdi.github.io/Guidelines/guide/qlever/)    |
+
+