Add README file for example downward experiment.

examples/downward/README.md

@@ -0,0 +1,23 @@
+# Setup instructions
+
+Create a [virtual environment](https://docs.python.org/3/tutorial/venv.html),
+activate it and install all dependencies:
+
+    sudo apt install python3 python3-venv
+    python3 -m venv --prompt myvenv .venv
+    source .venv/bin/activate
+    pip install --upgrade pip wheel
+    pip install -r requirements.txt
+
+If the last step fails, try regenerating a new `requirements.txt` from
+`requirements.in` for your Python version:
+
+    source .venv/bin/activate
+    pip install pip-tools
+    pip-compile
+    pip install -r requirements.txt
+
+Please note that before running an experiment script you need to
+activate the virtual environment with
+
+    source .venv/bin/activate