Update README.md

README.md

@@ -1,14 +1,17 @@
 # Hello World
-[![Join the chat at https://gitter.im/clarus/coq-hello-world](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/clarus/coq-hello-world?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+> A Hello World program in Coq.
 
-A Hello World program in Coq.
-
-## Run
-Install the extraction library for System effects:
+## Install from OPAM
+Make sure you added the [Coq repository](coq.io/opam/):
 
     opam repo add coq-released https://coq.inria.fr/opam/released
-    opam install -j4 coq:io:system
 
+and run:
+
+    opam install coq-io-hello-world
+    helloWorld
+
+## Install from source
 Compile the Coq code:
 
     ./configure.sh