demo: add scripts for cleaning leftover files

The demo expects its storage directory to be clean at start, otherwise undefined behavior will occur.

demo/README.rst

+Demo
+====
+
+Usage
+-----
+
+To run the demo, make sure all the dependencies are installed
+and run ``./demo 8000`` (or any port).
+
+Troubleshooting
+---------------
+
+If the demo crashes and fails to clean up its resources properly,
+run the following commands::
+
+  for b in $(sudo ip l | grep -Po 'NETNS\w\w[\d\-a-f]+'); do sudo ip l del $b; done
+  pkill screen
+  killall python
+  killall python3
+  find . -name '*.crt' -delete; find . -name '*.db' -delete; find . -name '*.log' -delete
+
+.. warning::
+
+    This will kill all Python processes. These commands assume you're running
+    the demo on a dedicated machine with nothing else on it.