Document tests/integration/_common.sh.

This makes it much easier to run certbot during development.

docs/contributing.rst

@@ -39,17 +39,17 @@ Then in each shell where you're working on the client, do:
 .. code-block:: shell
 
    source ./venv/bin/activate
+   . tests/integration/_common.sh
 
 After that, your shell will be using the virtual environment, and you run the
-client by typing:
-
-.. code-block:: shell
-
-   certbot
-
-Activating a shell in this way makes it easier to run unit tests
-with ``tox`` and integration tests, as described below. To reverse this, you
-can type ``deactivate``.  More information can be found in the `virtualenv docs`_.
+client by typing `certbot` or `certbot_test`. The latter is an alias that
+includes several flags useful for testing. For instance, it sets various output
+directories to point to /tmp/, and uses non-privileged ports for challenges, so
+root privileges are not required.
+
+Activating a shell with `venv/bin/activate` sets environment variables so that
+Python pulls in the correct versions of various packages needed by Certbot.
+More information can be found in the `virtualenv docs`_.
 
 .. _`virtualenv docs`: https://virtualenv.pypa.io