chore: add readme with link to docs

README.md

+# DTT Moodle Plugin
+
+This is the source code repository for the DTT Moodle Plugin.
+
+Documentation for this is done with GitLab Pages using MkDocs and is available here:  
+[https://transfer.hft-stuttgart.de/pages/dtt/][pages-link]
+
+If you don't want to open the Webpage or it is not available for whatever reason,
+the documentation root is found [here][docs-root]
+
+[docs-root]: /dtt/documentation/docs/index.md
+[pages-link]: https://transfer.hft-stuttgart.de/pages/dtt/