add instructions to use opam to install dependencies (#141)

* add instructions to use opam to install dependencies

* Fix wording.

---------

Co-authored-by: Théo Zimmermann <theo.zimmermann@telecom-paris.fr>

Author: ybertot

README.md.mustache

@@ -115,11 +115,15 @@ opam repo add rocq-released https://rocq-prover.org/opam/released
 opam install {{ opam_name }}{{^ opam_name }}coq-{{ shortname }}{{/ opam_name }}
 ```
 
-To instead build and install manually, do:
+To instead build and install manually, you need to make sure that all the
+libraries this development depends on are installed.  The easiest way to do that
+is still to rely on opam:
 
 ``` shell
 git clone {{# submodule }}--recurse-submodules {{/ submodule }}https://github.com/{{ organization }}/{{ shortname }}.git
 cd {{ shortname }}
+opam repo add rocq-released https://rocq-prover.org/opam/released
+opam install --deps-only .
 {{# dune }}
 dune build
 dune install