CONTRIBUTING: Include information on build dependencies

libvirt depends on a ton of packages, so trying to install them
all by using the classic approach of repeatedly running configure
and reacting to each failure by installing the corresponding
missing package will inevitably lead to frustration.

Luckily there's an easy solution to get most dependencies
installed in one fell swoop, and we just need to document it.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NLaine Stump <laine@redhat.com>

CONTRIBUTING.rst

@@ -17,3 +17,22 @@ your git clone run:
   $ make
 
 You'll find the freshly-built document in ``docs/contribute.html``.
+
+If ``configure`` fails because of missing dependencies, you can set
+up your system by calling
+
+::
+
+  $ sudo dnf builddep libvirt
+
+if you're on a RHEL-based distribution or
+
+::
+
+  $ sudo apt-get build-dep libvirt
+
+if you're on a Debian-based one.
+
+You might still be missing some dependencies if your distribution is
+shipping an old libvirt version, but that will get you much closer to
+where you need to be to build successfully from source.