doc: Add missing step to include Rocq repo when building from source

Author: TDiazT

INSTALL.md

@@ -85,11 +85,16 @@ This creates the `rocq.9.1` switch which initially contains only the
 basic `OCaml` `4.14.0` compiler with the `flambda` option enabled,
 and puts you in the right environment (check with `ocamlc -v`).
 
-Once in the right switch, you can install `Rocq` and the `Equations` package using:
+Once in the right switch, you can install `Rocq` and the `Equations` package.
+For that, you need to first add the Rocq packages repository, using:
+
+    # opam repo add rocq-released https://rocq-prover.org/opam/release, using:
+
+Then, to install the dependencies, it suffices to do:
 
     # opam install . --deps-only
 
-If the commands are successful you should have `coq` available (check with `coqc -v**).
+If the commands are successful you should have `rocq` available (check with `rocq -v**).
 
 
 **Remark:** You can create a [local switch](https://opam.ocaml.org/blog/opam-20-tips/#Local-switches) for