Update Readme

Author: diehlpk

README.md

@@ -11,5 +11,6 @@ The code for all exmaples is available here.
 Prerequirements:
 
 * [CMake](https://cmake.org/)
-* C++ compiler supporting C++ 17
+* C++ compiler supporting C++ 17. 
+  * For coroutine and sender/receiver a C++ compiler supporting C++ 20. 
 * [HPX](https://github.com/STEllAR-GROUP/hpx) (Optinoal)