Update README that the repo is not maintained

README.md

+# Warning: this repository is no longer maintained
+
+Please check out [GPFSL](https://gitlab.mpi-sws.org/iris/gpfsl), a betterment of iGPS.
+
 # iGPS & iRSL - encoding of GPS and RSL in Iris
 
 This repository contains the Coq development of **iGPS** and **iRSL**,