Update installation instructions.

Author: maranget

INSTALL.md

@@ -1,4 +1,4 @@
-With OPAM (recommended)
+Binary installation with OPAM
 =======================
 
 [Install OPAM](https://opam.ocaml.org/doc/Install.html), then:
@@ -10,7 +10,7 @@ Then, to get the newest version:
     % opam update
     % opam upgrade
 
-Without OPAM
+Source build
 ============
 
 Tools will be installed in PREFIX/bin, and various files in PREFIX/share/herdtools7.
@@ -21,10 +21,16 @@ Requirements
 ------------
 
 - OCaml (version >= 4.05.0)
-- dune*
+- dune
+- menhir (version >= 20180530)
 
-* Compilation with ocamlbuild is possible, by setting D=ocb in Makefile.
+We strongly recommend to have this base software installed through the opam
+package manager.
 
+    % opam install dune menhir
+
+Notice: Compilation with ocamlbuild is possible, by setting D=ocb in Makefile.
+ 
 Build
 -----
 

herdtools7.opam

@@ -20,5 +20,5 @@ install: ["sh" "./dune-install.sh" "%{prefix}%"]
 depends: [
   "ocaml" {>= "4.02.0"}
   "dune"
-  "menhir"
+  "menhir" {>= "20180530"}
 ]