add description to mermaid graph

README.md

@@ -4,6 +4,9 @@ Germann AT, Nakielski A, Dietsch M, Petzel T, Moser D, Triesch S, Westhoff P and
 ## Copyright
 Copyright © 2023 Germann, Nakielski, Dietsch, Petzel, Moser, Triesch, Westhoff and Axmann. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
 
+## ARC structure
+**green** indicates a study;
+**yellow** indicates an assay
 ``` mermaid
 flowchart LR
     p(publication)-->s1(Englund-2014)