Add first docs for Rattail DB, and POS / other data stores

docs/data/db/schema.rst

@@ -0,0 +1,18 @@
+
+Core Schema
+===========
+
+Rattail's core database schema contains all tables deemed "generic" enough to
+be potentially useful to "any" retail organization.  Many of the tables may go
+unused by a particular org, but they will all be present, if the DB exists.
+Some of the tables are truly generic, and some may be specific to Rattail
+itself.
+
+Assuming your DB is named "poser" you can see its table list like so:
+
+.. code-block:: sh
+
+   sudo -u postgres psql -c '\d' poser
+
+Note of course that this will show *all* tables in your DB, not just the core
+schema tables.