Update tut0.rst (#159)

* Update tut0.rst

Add hint about port mapping for users with pre-existing PostgreSQL DB

tutorials/tut0.rst

@@ -35,6 +35,16 @@ If Docker is not installed, you can get it `here <https://www.docker.com/communi
 
 This will run the Docker instance as a daemon and expose port 5432 to the host system so that it looks like an ordinary PostgreSQL server to the rest of the system.
 
+.. note::
+
+  If you have a pre-existing PostgreSQL database running on port 5432, you'll get an error like "bind: address already in use". Here's how to fix it:
+
+  .. code-block:: bash
+
+    sudo docker run --name tutorial -p 5433:5432 \
+                    -e POSTGRES_PASSWORD=mysecretpassword \
+                    -d postgres
+
 Step 3. Install PostgREST
 -------------------------