More precision about installation of ssreflect in the readme.

README.md

@@ -9,8 +9,13 @@ This version is known to compile with:
  - Coq 8.6
  - Ssreflect 1.6.1
 
-The easiest way to install the correct versions of the dependencies is through
-opam.  Once you got opam set up, just run `make build-dep` to install the right
+The easiest way to install the correct versions of the dependencies is
+through opam. Ssreflect is available on the coq-released repository,
+set up by the command:
+
+    opam repo add coq-released https://coq.inria.fr/opam/released
+
+Once you got opam set up, just run `make build-dep` to install the right
 versions of the dependencies.  When the dependencies change, just run `make
 build-dep` again.