CONTRIBUTING: Add entry point for new contributors

It's generally expected that a git repository will contain this file,
which serves as an entry point for people interested in contributing
to the project.

In our case, we have extensive documentation available on the
website which we don't want to duplicate, so let's just point people
there.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

CONTRIBUTING.rst

+=======================
+Contributing to libvirt
+=======================
+
+Full, up to date information on how to contribute to libvirt can be
+found on the libvirt website:
+
+https://libvirt.org/contribute.html
+
+To build the same document locally, from the top level directory of
+your git clone run:
+
+::
+
+  $ mkdir build && cd build
+  $ ../autogen.sh
+  $ make
+
+You'll find the freshly-built document in ``docs/contribute.html``.