hint in readme.md that kroki is not necessary for all graph types (not for...

hint in readme.md that kroki is not necessary for all graph types (not for mermaid, for that matter)

README.md

 # Create Diagrams from Textual Descriptions
 In this GitLab instance, you can easily create diagrams from textual descriptions.
-For this purpose, a tool called [Kroki](https://kroki.io) is provided.
+For most diagram types, a tool called [Kroki](https://kroki.io) is provided.
 The integration allows the implementation of a wide range of creative ideas
 in diagrams and visualizations.