Re-insert known issue for rdflib version in README

README.md

@@ -140,7 +140,14 @@ for how to build a docker image.
 Known issues
 ------------
 
-No known issues at this point.
+* **Invalid serialising to turtle:** Due to rdflib issue
+  [#1043](https://github.com/RDFLib/rdflib/issues/1043) ontoconvert
+  may produce invalid turtle output (if your ontology contains real
+  literals using scientific notation without a dot in the mantissa).
+  This issue was fixed after the release of rdflib 5.0.0.  Hence,
+  install rdflib from [github
+  master](https://github.com/RDFLib/rdflib) if you need to serialise
+  to turtle.
 
 Attributions and credits
 ------------------------