display example code and usage instructions in docs

docs/examples.echo.rst

@@ -0,0 +1,18 @@
+Echo Demo
+=========
+
+Copy the code below into a file called ``demo.py``.
+Install dependencies, preferably in a virtual environment, with:
+
+.. code-block:: bash
+
+    python -m pip install libp2p
+
+Run the demo with ``python echo.py`` and copy the output.
+
+Open a second terminal, navigate to the folder that contains ``echo.py``, then paste
+and run the copied line.
+
+.. literalinclude:: ../examples/echo/echo.py
+    :language: python
+    :linenos: