Readme: Mention why we suggest using sudo

README.rst

@@ -36,6 +36,9 @@ The system tests are run using this command::
   $ sudo --preserve-env nix-build nixos/system-tests.nix
 
 The system tests boot QEMU VMs which prevents them from running on CI at this time.
+Starting VMs require elevated privileges,
+which is the reason for the suggested ``sudo`` command,
+together with the ``--preserve-env`` option to avoid escaping the ``nix-shell`` set up at the top.
 The build requires > 10 GB of disk space, and the VMs might be timing out on slow or busy machines.
 If you run into timeouts, try `raising the number of retries <https://github.com/PrivateStorageio/PrivateStorageio/blob/e8233d2/nixos/modules/tests/run-introducer.py#L55-L62>`_.