update README

README.md

@@ -11,8 +11,10 @@ This version is known to compile with:
  - A development version of [Iris](https://gitlab.mpi-sws.org/FP/iris-coq/)
  - The coq86-devel branch of [Autosubst](https://github.com/uds-psl/autosubst)
 
-The easiest way to install the correct versions of the dependencies is through
-opam.  You will need the Coq and Iris opam repositories:
+## Building from source
+
+When building from source, we recommend to use opam (1.2.2 or newer) for
+installing the dependencies.  This requires the following two repositories:
 
     opam repo add coq-released https://coq.inria.fr/opam/released
     opam repo add iris-dev https://gitlab.mpi-sws.org/FP/opam-dev.git
@@ -20,9 +22,12 @@ opam.  You will need the Coq and Iris opam repositories:
 Once you got opam set up, run `make build-dep` to install the right versions
 of the dependencies.
 
-## Building
+Run `make -jN` to build the full development, where `N` is the number of your
+CPU cores.
 
-Run `make` to build the full development.
+To update, do `git pull`.  After an update, the development may fail to compile
+because of outdated dependencies.  To fix that, please run `opam update`
+followed by `make build-dep`.
 
 ## Case Studies