document packages that are necessary to make a release

CONTRIBUTING.rst

@@ -374,6 +374,15 @@ A workaround is to hardcode the version with::
 Release process
 ===============
 
+To build a Monkeysign release, you will need to have a few tools
+already installed, namely the Python packages ``wheel``,
+``setuptools`` and ``setuptools-scm``. We also assume you use the
+following Debian packages, although you may be able to work around
+those: ``devscripts``, ``git``, ``git-buildpackage``, ``pip`` and
+``twine``. In Debian, this should get you started::
+
+  sudo apt install python-wheel python-setuptools python-setuptools-scm devscripts git git-buildpackage python-pip twine
+
 1. make sure tests pass::
 
      ./scripts/monkeysign --test