more README changes

README.md

@@ -49,6 +49,10 @@ repository:
 
 Then you can do `opam install coq-stdpp`.
 
+To obtain a development version, add the Iris opam repository:
+
+    opam repo add iris-dev https://gitlab.mpi-sws.org/FP/opam-dev.git
+
 ## Building from source
 
 Run `make -jN` in this directory to build the library, where `N` is the number