Add basic database docs

docs/narr/db/app.rst

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+
+App Database
+============
+
+The :term:`app database` is used at minimum to contain the
+:term:`settings table`.
+
+There is not yet support within WuttJamaican for the creation or setup
+of the app database.  So for now you're on your own with that.
+
+See also :doc:`/narr/config/table`.
+
+Note that while any database supported by SQLAlchemy may be used, docs
+will generally assume PostgreSQL is being used.
+
+
+Configuring the Connection
+--------------------------
+
+Once you have a database ready, add to your :term:`config file` the
+details, for example:
+
+.. code-block:: ini
+
+   [wutta.db]
+   default.url = postgresql://wutta:wuttapass@localhost/wuttadb
+
+
+Multiple Databases
+------------------
+
+Some scenarios may require multiple app databases.  A notable example
+would be a multi-store retail environment, where each store runs a
+separate app but a "host" (master) node has connections to all store
+databases.
+
+Using that example, the host config might look like:
+
+.. code-block:: ini
+
+   [wutta.db]
+   # nb. host itself is referred to as 'default'
+   keys = default, store001, store002, store003
+
+   default.url = postgresql://wutta:wuttapos@localhost/wutta-host
+
+   store001.url = postgresql://wutta:wuttapos@store001/wutta-store
+   store002.url = postgresql://wutta:wuttapos@store002/wutta-store
+   store003.url = postgresql://wutta:wuttapos@store003/wutta-store