Add docs on how to use synapse with psycopg2

docs/postgres.rst

+Using Postgres
+--------------
+
+Set up client
+=============
+We need to have installed the postgres python connector ``psycopg2``. In the
+virtual env::
+
+    sudo apt-get install libpq-dev
+    pip install psycopg2
+
+
+Synapse config
+==============
+
+Add the following line to your config file::
+
+    database_config: <db_config_file>
+
+Where ``<db_config_file>`` is the file name that points to a yaml file of the
+following form::
+
+    name: psycopg2
+    args:
+        user: <user>
+        password: <pass>
+        database: <db>
+        host: <host>
+        cp_min: 5
+        cp_max: 10
+
+All key, values in ``args`` are passed to the ``psycopg2.connect(..)``
+function, except keys beginning with ``cp_``, which are consumed by the twisted
+adbapi connection pool.