add README

README.md

+# PNML for Relational RAGs
+
+A tool to read PNML XML files and transform them into ASTs of a relational RAG.
+
+The tool supports place-transition nets as defined in [ISO/IEC 15909-1:2019](https://www.iso.org/standard/67235.html).
+
+Reading is done using the [PNML Framework](https://pnml.lip6.fr/).
+
+The grammar is constructed automatically from the [ecore model](http://www.pnml.org/grammar.php) provided by [Pnml.org](http://www.pnml.org/).